diff --git a/app/controllers/api/v1/App.controller.js b/app/controllers/api/v1/App.controller.js
index f0435fe..145b417 100644
--- a/app/controllers/api/v1/App.controller.js
+++ b/app/controllers/api/v1/App.controller.js
@@ -25,12 +25,12 @@ class AppController extends Controller {
 
         if ( !application || !application.active )
             return res.status(404)
-                .message('Application not found with that ID.')
+                .message(req.T('api:application_not_found'))
                 .api()
 
         if ( !req.user.can(`app:${application.id}:view`) )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         return res.api(await application.to_api())
@@ -41,28 +41,28 @@ class AppController extends Controller {
 
         if ( !req.user.can('app:create') )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         const required_fields = ['name', 'identifier']
         for ( const field of required_fields ) {
             if ( !req.body[field] )
                 return res.status(400)
-                    .message(`Missing required field: ${field}`)
+                    .message(`${req.T('api:missing_field')} ${field}`)
                     .api()
         }
 
         // Make sure the identifier is properly formatted
         if ( !(new RegExp('^[a-zA-Z0-9_]*$')).test(req.body.identifier) )
             return res.status(400)
-                .message('Improperly formatted field: identifier (alphanumeric with underscores only)')
+                .message(`${req.T('api:improper_field')} identifier ${req.T('api:alphanum_underscores')}`)
                 .api()
 
         // Make sure the identifier is unique
         const existing_app = await Application.findOne({ identifier: req.body.identifier })
         if ( existing_app )
             return res.status(400)
-                .message('An Application with that identifier already exists.')
+                .message(req.T('api:application_already_exists'))
                 .api()
 
         const application = new Application({
@@ -80,12 +80,12 @@ class AppController extends Controller {
                 const client = await LDAPClient.findById(id)
                 if ( !client || !client.active || !req.user.can(`ldap:client:${client.id}:view`) )
                     return res.status(400)
-                        .message(`Invalid ldap_client_id: ${id}`)
+                        .message(`${req.T('api:invalid_ldap_client_id')} ${id}`)
                         .api()
 
                 const other_assoc_app = await Application.findOne({ ldap_client_ids: client.id })
                 if ( other_assoc_app )
-                    return res.status(400)
+                    return res.status(400) // TODO translate this
                         .message(`The LDAP client ${client.name} is already associated with an existing application (${other_assoc_app.name}).`)
                         .api()
             }
@@ -102,12 +102,12 @@ class AppController extends Controller {
                 const client = await OAuthClient.findById(id)
                 if ( !client || !client.active || !req.user.can(`oauth:client:${client.id}:view`) )
                     return res.status(400)
-                        .message(`Invalid oauth_client_id: ${id}`)
+                        .message(`${req.T('api:invalid_oauth_client_id')} ${id}`)
                         .api()
 
                 const other_assoc_app = await Application.findOne({ oauth_client_ids: client.id })
                 if ( other_assoc_app )
-                    return res.status(400)
+                    return res.status(400) // TODO translate this
                         .message(`The OAuth2 client ${client.name} is already associated with an existing application (${other_assoc_app.name}).`)
                         .api()
             }
@@ -124,12 +124,12 @@ class AppController extends Controller {
                 const provider = await ServiceProvider.findById(id)
                 if ( !provider || !provider.active || !req.user.can(`saml:provider:${provider.id}:view`) )
                     return res.status(400)
-                        .message(`Invalid saml_service_provider_id: ${id}`)
+                        .message(`${req.T('api:invalid_saml_service_provider_id')} ${id}`)
                         .api()
 
                 const other_assoc_app = await Application.findOne({ saml_service_provider_ids: provider.id })
                 if ( other_assoc_app )
-                    return res.status(400)
+                    return res.status(400) // TODO translate this
                         .message(`The SAML service provider ${provider.name} is already associated with an existing application (${other_assoc_app.name}).`)
                         .api()
             }
@@ -147,33 +147,33 @@ class AppController extends Controller {
 
         if ( !application || !application.active )
             return res.status(404)
-                .message('Application not found with that ID.')
+                .message(req.T('api:application_not_found'))
                 .api()
 
         if ( !req.user.can(`app:${application.id}:update`) )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         const required_fields = ['name', 'identifier']
         for ( const field of required_fields ) {
             if ( !req.body[field] )
                 return res.status(400)
-                    .message(`Missing required field: ${field}`)
+                    .message(`${req.T('api:missing_field')} ${field}`)
                     .api()
         }
 
         // Make sure the identifier is properly formatted
         if ( !(new RegExp('^[a-zA-Z0-9_]*$')).test(req.body.identifier) )
             return res.status(400)
-                .message('Improperly formatted field: identifier (alphanumeric with underscores only)')
+                .message(`${req.T('api:improper_field')} identifier ${req.T('api:alphanum_underscores')}`)
                 .api()
 
         // Make sure the identifier is unique
         const existing_app = await Application.findOne({ identifier: req.body.identifier })
         if ( existing_app && existing_app.id !== application.id )
             return res.status(400)
-                .message('An Application with that identifier already exists.')
+                .message(req.T('api:application_already_exists'))
                 .api()
 
         // Verify LDAP client IDs
@@ -185,12 +185,12 @@ class AppController extends Controller {
                 const client = await LDAPClient.findById(id)
                 if ( !client || !client.active || !req.user.can(`ldap:client:${client.id}:view`) )
                     return res.status(400)
-                        .message(`Invalid ldap_client_id: ${id}`)
+                        .message(`${req.T('api:invalid_ldap_client_id')} ${id}`)
                         .api()
 
                 const other_assoc_app = await Application.findOne({ ldap_client_ids: client.id })
                 if ( other_assoc_app && other_assoc_app.id !== application.id )
-                    return res.status(400)
+                    return res.status(400) // TODO translate this
                         .message(`The LDAP client ${client.name} is already associated with an existing application (${other_assoc_app.name}).`)
                         .api()
             }
@@ -207,12 +207,12 @@ class AppController extends Controller {
                 const client = await OAuthClient.findById(id)
                 if ( !client || !client.active || !req.user.can(`oauth:client:${client.id}:view`) )
                     return res.status(400)
-                        .message(`Invalid oauth_client_id: ${id}`)
+                        .message(`${req.T('api:invalid_oauth_client_id')} ${id}`)
                         .api()
 
                 const other_assoc_app = await Application.findOne({ oauth_client_ids: client.id })
                 if ( other_assoc_app && other_assoc_app.id !== application.id )
-                    return res.status(400)
+                    return res.status(400) // TODO translate this
                         .message(`The OAuth2 client ${client.name} is already associated with an existing application (${other_assoc_app.name}).`)
                         .api()
             }
@@ -229,12 +229,12 @@ class AppController extends Controller {
                 const provider = await ServiceProvider.findById(id)
                 if ( !provider || !provider.active || !req.user.can(`saml:provider:${provider.id}:view`) )
                     return res.status(400)
-                        .message(`Invalid saml_service_provider_id: ${id}`)
+                        .message(`${req.T('api:invalid_saml_service_provider_id')} ${id}`)
                         .api()
 
                 const other_assoc_app = await Application.findOne({ saml_service_provider_ids: provider.id })
                 if ( other_assoc_app && other_assoc_app.id !== application.id )
-                    return res.status(400)
+                    return res.status(400) // TODO translate this
                         .message(`The SAML service provider ${provider.name} is already associated with an existing application (${other_assoc_app.name}).`)
                         .api()
             }
@@ -255,12 +255,12 @@ class AppController extends Controller {
 
         if ( !application || !application.active )
             return res.status(404)
-                .message('Application not found with that ID.')
+                .message(req.T('api:application_not_found'))
                 .api()
 
         if ( !req.user.can(`app:${application.id}:delete`) )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         application.active = false
diff --git a/app/controllers/api/v1/Auth.controller.js b/app/controllers/api/v1/Auth.controller.js
index 9930ff3..a2fa8da 100644
--- a/app/controllers/api/v1/Auth.controller.js
+++ b/app/controllers/api/v1/Auth.controller.js
@@ -20,7 +20,7 @@ class AuthController extends Controller {
 
     async get_traps(req, res, next) {
         const trap_config = this.configs.get('traps')
-        const data = [{ name: '(None)', trap: '', redirect_to: '/' }]
+        const data = [{ name: req.T('auth:none'), trap: '', redirect_to: '/' }]
         for ( const name in trap_config.types ) {
             if ( !trap_config.types.hasOwnProperty(name) ) continue
             data.push({
@@ -44,18 +44,18 @@ class AuthController extends Controller {
         for ( const field of required_fields ) {
             if ( !req.body[field] )
                 return res.status(400)
-                    .message(`Missing required field: ${field}`)
+                    .message(`${req.T('api:missing_field')} ${field}`)
                     .api()
         }
 
         if ( !req.body.uid.match(/^([A-Z]|[a-z]|[0-9]|_|-|\.)+$/) )
             return res.status(400)
-                .message('Invalid field: uid (should be alphanumeric with "_", "-", and "." allowed)')
+                .message(`${req.T('api:improper_field')} uid ${req.T('api:alphanum_underscores')}`)
                 .api()
 
         if ( !email_validator.validate(req.body.email) )
             return res.status(400)
-                .message('Invalid field: email')
+                .message(`${req.T('api:improper_field')} email`)
                 .api()
 
         for ( const field of unique_fields ) {
@@ -64,7 +64,7 @@ class AuthController extends Controller {
             const match_user = await User.findOne(params)
             if ( match_user )
                 return res.status(400)
-                    .message(`A user already exists with that ${field}.`)
+                    .message(`${req.T('auth:user_exists_with_field')} ${field}`)
                     .api()
         }
 
@@ -91,12 +91,12 @@ class AuthController extends Controller {
             || req.user.mfa_token.recovery_codes.length < 1
         )
             return res.status(400)
-                .message('Your user is not configured to use MFA, or has no recovery codes.')
+                .message(req.T('auth:no_mfa_or_recovery'))
                 .api()
 
         if ( !req.body.code )
             return res.status(400)
-                .message('Missing required field: code')
+                .message(`${req.T('api:missing_field')} code`)
                 .api()
 
         const success = await req.user.mfa_token.attempt_recovery(req.body.code)
@@ -176,7 +176,7 @@ class AuthController extends Controller {
 
         if ( !group || !group.active )
             return res.status(404)
-                .message('Group not found with that ID.')
+                .message(req.T('api:group_not_found'))
                 .api()
 
         if ( !req.user.can(`auth:group:${group.id}:view`) )
@@ -196,12 +196,12 @@ class AuthController extends Controller {
 
         if ( !user )
             return res.status(404)
-                .message('User not found with that ID.')
+                .message(req.T('api:user_not_found'))
                 .api()
 
         if ( !req.user.can(`auth:user:${user.id}:view`) )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         return res.api(await user.to_api())
@@ -210,12 +210,12 @@ class AuthController extends Controller {
     async create_group(req, res, next) {
         if ( !req.user.can(`auth:group:create`) )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         if ( !req.body.name )
             return res.status(400)
-                .message('Missing required field: name')
+                .message(`${req.T('api:missing_field')} name`)
                 .api()
 
         const Group = this.models.get('auth:Group')
@@ -224,7 +224,7 @@ class AuthController extends Controller {
         const existing_group = await Group.findOne({ name: req.body.name })
         if ( existing_group )
             return res.status(400)
-                .message('A group with that name already exists.')
+                .message(req.T('api:group_already_exists'))
                 .api()
 
         const group = new Group({ name: req.body.name })
@@ -238,7 +238,7 @@ class AuthController extends Controller {
                 const user = await User.findById(user_id)
                 if ( !user )
                     return res.status(400)
-                        .message('Invalid user_id.')
+                        .message(`${req.T('common:invalid')} user_id.`)
                         .api()
             }
 
@@ -252,14 +252,14 @@ class AuthController extends Controller {
     async create_user(req, res, next) {
         if ( !req.user.can('auth:user:create') )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         const required_fields = ['uid', 'first_name', 'last_name', 'email', 'password']
         for ( const field of required_fields ) {
             if ( !req.body[field] )
                 return res.status(400)
-                    .message(`Missing required field: ${field}`)
+                    .message(`${req.T('api:missing_field')} ${field}`)
                     .api()
         }
 
@@ -272,7 +272,7 @@ class AuthController extends Controller {
             const existing_user = await User.findOne(filter)
             if ( existing_user )
                 return res.status(400)
-                    .message(`A user already exists with that ${field}`)
+                    .message(`${req.T('auth:user_exists_with_field')} ${field}`)
                     .api()
         }
 
@@ -281,7 +281,7 @@ class AuthController extends Controller {
         const result = zxcvbn(req.body.password)
         if ( result.score < min_score )
             return res.status(400)
-                .message(`Password does not meet the minimum complexity score of ${min_score}.`)
+                .message(req.T('auth:password_complexity_fail').replace('MIN_SCORE', min_score))
                 .api()
 
         const user = new User({
@@ -297,7 +297,7 @@ class AuthController extends Controller {
         if ( req.body.trap ) {
             if ( !req.trap.trap_exists(req.body.trap) )
                 return res.status(400)
-                    .message('Invalid trap type.')
+                    .message(req.T('auth:invalid_trap'))
                     .api()
 
             user.trap = req.body.trap
@@ -315,24 +315,24 @@ class AuthController extends Controller {
         const group = await Group.findById(req.params.id)
         if ( !group )
             return res.status(404)
-                .message('Group not found with that ID.')
+                .message(req.T('api:group_not_found'))
                 .api()
 
         if ( !req.user.can(`auth:group:${group.id}:update`) )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         if ( !req.body.name )
             return res.status(400)
-                .message('Missing required field: name')
+                .message(`${req.T('api:missing_field')} name`)
                 .api()
 
         // Make sure the group name is unique
         const existing_group = await Group.findOne({ name: req.body.name })
         if ( existing_group && existing_group.id !== group.id )
             return res.status(400)
-                .message('A group with that name already exists.')
+                .message(req.T('api:group_already_exists'))
                 .api()
 
         // Validate user_ids
@@ -343,7 +343,7 @@ class AuthController extends Controller {
                 const user = await User.findById(user_id)
                 if ( !user )
                     return res.status(400)
-                        .message('Invalid user_id.')
+                        .message(`${req.T('common:invalid')} user_id.`)
                         .api()
             }
 
@@ -363,19 +363,19 @@ class AuthController extends Controller {
 
         if ( !user )
             return res.status(404)
-                .message('User not found with that ID.')
+                .message(req.T('api:user_not_found'))
                 .api()
 
         if ( !req.user.can(`auth:user:${user.id}:update`) )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         const required_fields = ['uid', 'first_name', 'last_name', 'email']
         for ( const field of required_fields ) {
             if ( !req.body[field] )
                 return res.status(400)
-                    .message(`Missing required field: ${field}`)
+                    .message(`${req.T('api:missing_field')} ${field}`)
                     .api()
         }
 
@@ -387,7 +387,7 @@ class AuthController extends Controller {
             const existing_user = await User.findOne(filter)
             if ( existing_user && existing_user.id !== user.id )
                 return res.status(400)
-                    .message(`A user already exists with that ${field}`)
+                    .message(`${req.T('auth:user_exists_with_field')} ${field}`)
                     .api()
         }
 
@@ -397,7 +397,7 @@ class AuthController extends Controller {
             const result = zxcvbn(req.body.password)
             if (result.score < min_score)
                 return res.status(400)
-                    .message(`Password does not meet the minimum complexity score of ${min_score}.`)
+                    .message(req.T('auth:password_complexity_fail').replace('MIN_SCORE', min_score))
                     .api()
 
             await user.reset_password(req.body.password, 'api')
@@ -416,7 +416,7 @@ class AuthController extends Controller {
         if ( req.body.trap ) {
             if ( !req.trap.trap_exists(req.body.trap) )
                 return res.status(400)
-                    .message('Invalid trap type.')
+                    .message(req.T('auth:invalid_trap'))
                     .api()
 
             user.trap = req.body.trap
@@ -433,12 +433,12 @@ class AuthController extends Controller {
 
         if ( !group )
             return res.status(404)
-                .message('Group not found with that ID.')
+                .message(req.T('api:group_not_found'))
                 .api()
 
         if ( !req.user.can(`auth:group:${group.id}:delete`) )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         group.active = false
@@ -452,12 +452,12 @@ class AuthController extends Controller {
 
         if ( !user )
             return res.status(404)
-                .message('User not found with that ID.')
+                .message(req.T('api:user_not_found'))
                 .api()
 
         if ( !req.user.can(`auth:user:${user.id}:delete`) )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         // check if the user is an LDAP client. if so, delete the client
@@ -493,7 +493,7 @@ class AuthController extends Controller {
 
         if ( !req.body.username && !req.body.email )
             return res.status(400)
-                .message('Please provide one of: username, email')
+                .message(`${req.T('api:provide_one')} username, email`)
                 .api()
 
         const data = {}
@@ -529,7 +529,7 @@ class AuthController extends Controller {
         const errors = await flitter.validate_login(req.body)
         if ( errors && errors.length > 0 )
             return res.status(400)
-                .message(`Unable to complete authentication: one or more errors occurred`)
+                .message(req.T('auth:unable_to_complete'))
                 .api({ errors })
 
         const login_args = await flitter.get_login_args(req.body)
@@ -537,9 +537,9 @@ class AuthController extends Controller {
 
         if ( !user )
             return res.status(200)
-                .message(`Invalid username or password.`)
+                .message(req.T('auth:invalid_un_or_pw'))
                 .api({
-                    message: `Invalid username or password.`,
+                    message: req.T('auth:invalid_un_or_pw'),
                     success: false,
                 })
 
@@ -549,9 +549,9 @@ class AuthController extends Controller {
         const client = await Client.findOne({ user_id: user.id })
         if ( client )
             return res.status(200)
-                .message(`Invalid username or password.`)
+                .message(req.T('auth:invalid_un_or_pw'))
                 .api({
-                    message: `Invalid username or password.`,
+                    message: req.T('auth:invalid_un_or_pw'),
                     success: false,
                 })
 
@@ -585,7 +585,7 @@ class AuthController extends Controller {
                 }
             } else {
                 return res.status(401)
-                    .message(`Unable to grant trust. Grant token is invalid.`)
+                    .message(req.T('auth:unable_to_grant_trust'))
                     .api()
             }
         }
@@ -600,7 +600,7 @@ class AuthController extends Controller {
     async get_mfa_recovery(req, res, next) {
         if ( !req.user.mfa_enabled )
             return res.status(400)
-                .message('Your user does not have MFA enabled.')
+                .message(req.T('auth:no_mfa'))
                 .api()
 
         const token = req.user.mfa_token
@@ -617,7 +617,7 @@ class AuthController extends Controller {
     async generate_mfa_recovery(req, res, next) {
         if ( !req.user.mfa_enabled )
             return res.status(400)
-                .message('Your user does not have MFA enabled.')
+                .message(req.T('auth:no_mfa'))
                 .api()
 
         const token = req.user.mfa_token
@@ -631,7 +631,7 @@ class AuthController extends Controller {
     async generate_mfa_key(req, res, next) {
         if ( req.user.mfa_enabled )
             return res.status(400)
-                .message(`MFA already configured for user. Cannot fetch key.`)
+                .message(req.T('auth:already_has_mfa'))
                 .api()
 
         const MFAToken = this.models.get('auth:MFAToken')
@@ -654,7 +654,7 @@ class AuthController extends Controller {
     async attempt_mfa(req, res, next) {
         if ( !req.user.mfa_token )
             return res.status(400)
-                .message(`The user does not have MFA configured.`)
+                .message(req.T('auth:no_mfa'))
                 .api()
 
         const code = req.body.verify_code
@@ -682,7 +682,7 @@ class AuthController extends Controller {
     async enable_mfa(req, res, next) {
         if ( !req.user.mfa_token )
             return res.status(400)
-                .message(`The user does not have an MFA token configured.`)
+                .message(req.T('auth:no_mfa'))
                 .api()
 
         req.user.mfa_enabled = true
@@ -700,7 +700,7 @@ class AuthController extends Controller {
     async disable_mfa(req, res, next) {
         if ( !req.user.mfa_enabled )
             return res.status(400)
-                .message('The user does not have MFA enabled.')
+                .message(req.T('auth:no_mfa'))
                 .api()
 
         req.user.mfa_enabled = false
diff --git a/app/controllers/api/v1/IAM.controller.js b/app/controllers/api/v1/IAM.controller.js
index 3276e5f..53e4fad 100644
--- a/app/controllers/api/v1/IAM.controller.js
+++ b/app/controllers/api/v1/IAM.controller.js
@@ -10,7 +10,7 @@ class IAMController extends Controller {
 
         if ( !req.body.entity_id && !req.body.target_id )
             return res.status(400)
-                .message('Missing one or more required fields: entity_id, target_id')
+                .message(`${req.T('api:missing_field', true)} entity_id, target_id`)
                 .api()
 
         return res.api(await Policy.check_entity_access(req.body.entity_id, req.body.target_id))
@@ -22,7 +22,7 @@ class IAMController extends Controller {
 
         if ( !req.body.target_id )
             return res.status(400)
-                .message('Missing required field: target_id')
+                .message(`${req.T('api:missing_field')} target_id`)
                 .api()
 
         let user = req.user
@@ -31,12 +31,12 @@ class IAMController extends Controller {
 
         if ( !user )
             return res.status(404)
-                .message('User not found with that ID.')
+                .message(req.T('api:user_not_found'))
                 .api()
 
         if ( !req.user.can(`auth:user:${user.id}:view`) )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         return res.api(await Policy.check_user_access(user, req.body.target_id))
@@ -62,12 +62,12 @@ class IAMController extends Controller {
 
         if ( !policy )
             return res.status(404)
-                .message('Policy not found with that ID.')
+                .message(req.T('iam:policy_not_found'))
                 .api()
 
         if ( !req.user.can(`iam:policy:${policy.id}:view`) )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         return res.api(await policy.to_api())
@@ -80,13 +80,13 @@ class IAMController extends Controller {
         for ( const field of required_fields ) {
             if ( !req.body[field] )
                 return res.status(400)
-                    .message(`Missing required field: ${field}`)
+                    .message(`${req.T('api:missing_field')} ${field}`)
                     .api()
         }
 
         if ( !['user', 'group'].includes(req.body.entity_type) )
             return res.status(400)
-                .message('Invalid entity_type. Must be one of: user, group.')
+                .message(`${req.T('iam:invalid_entity')} user, group`)
                 .api()
 
         // Make sure the entity_id is valid
@@ -95,25 +95,25 @@ class IAMController extends Controller {
             const user = await User.findById(req.body.entity_id)
             if ( !user || !req.user.can(`auth:user:${user.id}:view`) )
                 return res.status(400)
-                    .message('Invalid entity_id.')
+                    .message(`${req.T('common:invalid')} entity_id.`)
                     .api()
         } else if ( req.body.entity_type === 'group' ) {
             const Group = this.models.get('auth:Group')
             const group = await Group.findById(req.body.entity_id)
             if ( !group || !group.active || !req.user.can(`auth:group:${group.id}:view`) )
                 return res.status(400)
-                    .message('Invalid entity_id.')
+                    .message(`${req.T('common:invalid')} entity_id.`)
                     .api()
         }
 
         if ( !['allow', 'deny'].includes(req.body.access_type) )
             return res.status(400)
-                .message('Invalid access_type. Must be one of: allow, deny.')
+                .message(`${req.T('common:invalid')} access_type. ${req.T('api:must_one')} allow, deny.`)
                 .api()
 
         if ( !['application', 'api_scope'].includes(req.body.target_type) )
             return res.status(400)
-                .message('Invalid target_type. Must be one of: application.')
+                .message(`${req.T('common:invalid')} target_type. ${req.T('api:must_one')} application, api_scope.`)
                 .api()
 
         // Make sure the target_id is valid
@@ -122,13 +122,13 @@ class IAMController extends Controller {
             const app = await Application.findById(req.body.target_id)
             if ( !app || !app.active || !req.user.can(`app:${app.id}:view`) )
                 return res.status(400)
-                    .message('Invalid target_id.')
+                    .message(`${req.T('common:invalid')} target_id.`)
                     .api()
         } else if ( req.body.target_type === 'api_scope' ) {
             const api_scopes = this.canon.get('controller::api:v1:Reflect.api_scopes')()
             if ( !api_scopes.includes(req.body.target_id) )
                 return res.status(400)
-                    .message('Invalid target_id.')
+                    .message(`${req.T('common:invalid')} target_id.`)
                     .api()
         }
 
@@ -152,25 +152,25 @@ class IAMController extends Controller {
 
         if ( !policy || !policy.active )
             return res.status(404)
-                .message('Policy not found with that ID.')
+                .message(req.T('iam:policy_not_found'))
                 .api()
 
         if ( !req.user.can(`iam:policy:${policy.id}:update`) )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         const required_fields = ['entity_type', 'entity_id', 'access_type', 'target_type', 'target_id']
         for ( const field of required_fields ) {
             if ( !req.body[field] )
                 return res.status(400)
-                    .message(`Missing required field: ${field}`)
+                    .message(`${req.T('api:missing_field')} ${field}`)
                     .api()
         }
 
         if ( !['user', 'group'].includes(req.body.entity_type) )
             return res.status(400)
-                .message('Invalid entity_type. Must be one of: user, group.')
+                .message(`${req.T('common:invalid')} entity_type. ${req.T('api:must_one')} user, group.`)
                 .api()
 
         // Make sure the entity_id is valid
@@ -179,25 +179,25 @@ class IAMController extends Controller {
             const user = await User.findById(req.body.entity_id)
             if ( !user || !req.user.can(`auth:user:${user.id}:view`) )
                 return res.status(400)
-                    .message('Invalid entity_id.')
+                    .message(`${req.T('common:invalid')} entity_id.`)
                     .api()
         } else if ( req.body.entity_type === 'group' ) {
             const Group = this.models.get('auth:Group')
             const group = await Group.findById(req.body.entity_id)
             if ( !group || !group.active || !req.user.can(`auth:group:${group.id}:view`) )
                 return res.status(400)
-                    .message('Invalid entity_id.')
+                    .message(`${req.T('common:invalid')} entity_id.`)
                     .api()
         }
 
         if ( !['allow', 'deny'].includes(req.body.access_type) )
             return res.status(400)
-                .message('Invalid access_type. Must be one of: allow, deny.')
+                .message(`${req.T('common:invalid')} access_type. ${req.T('api:must_one')} allow, deny.`)
                 .api()
 
         if ( !['application', 'api_scope'].includes(req.body.target_type) )
             return res.status(400)
-                .message('Invalid target_type. Must be one of: application.')
+                .message(`${req.T('common:invalid')} target_type. ${req.T('api:must_one')} application, api_scope.`)
                 .api()
 
         // Make sure the target_id is valid
@@ -206,13 +206,13 @@ class IAMController extends Controller {
             const app = await Application.findById(req.body.target_id)
             if ( !app || !app.active || !req.user.can(`app:${app.id}:view`) )
                 return res.status(400)
-                    .message('Invalid target_id.')
+                    .message(`${req.T('common:invalid')} target_id.`)
                     .api()
         } else if ( req.body.target_type === 'api_scope' ) {
             const api_scopes = this.canon.get('controller::api:v1:Reflect.api_scopes')()
             if ( !api_scopes.includes(req.body.target_id) )
                 return res.status(400)
-                    .message('Invalid target_id.')
+                    .message(`${req.T('common:invalid')} target_id.`)
                     .api()
         }
 
@@ -231,12 +231,12 @@ class IAMController extends Controller {
 
         if ( !policy || !policy.active )
             return res.status(404)
-                .message('Policy not found with that ID.')
+                .message(req.T('iam:policy_not_found'))
                 .api()
 
         if ( !req.user.can(`iam:policy:${policy.id}:delete`) )
             return res.status(401)
-                .message('Insufficient permissions.')
+                .message(req.T('api:insufficient_permissions'))
                 .api()
 
         policy.active = false
diff --git a/locale/en_US/api.locale.js b/locale/en_US/api.locale.js
new file mode 100644
index 0000000..87f0f04
--- /dev/null
+++ b/locale/en_US/api.locale.js
@@ -0,0 +1,23 @@
+module.exports = exports = {
+    application_not_found: 'Application not found with that ID.',
+    application_already_exists: 'An Application with that identifier already exists.',
+
+    group_not_found: 'Group not found with that ID.',
+    group_already_exists: 'A group with that name already exists.',
+
+    user_not_found: 'User not found with that ID.',
+
+    invalid_ldap_client_id: 'Invalid ldap_client_id:',
+    invalid_oauth_client_id: 'Invalid oauth_client_id:',
+    invalid_saml_service_provider_id: 'Invalid saml_service_provider_id:',
+
+    insufficient_permissions: 'Insufficient permissions.',
+    missing_field: {
+        one: 'Missing required field:',
+        many: 'Missing one or more required fields:',
+    },
+    improper_field: 'Improperly formatted field:',
+    alphanum_underscores: '(alphanumeric/underscores)',
+    provide_one: 'Please provide one of:',
+    must_one: 'Must be one of:',
+}
diff --git a/locale/en_US/auth.locale.js b/locale/en_US/auth.locale.js
new file mode 100644
index 0000000..4fbef18
--- /dev/null
+++ b/locale/en_US/auth.locale.js
@@ -0,0 +1,14 @@
+module.exports = exports = {
+    none: '(None)',
+    user_exists_with_field: 'A user already exists with the field: ',
+    no_mfa_or_recovery: 'Your user is not configured to use MFA, or has no recovery codes.',
+    no_mfa: 'Your user is not configured to use MFA.',
+    already_has_mfa: 'MFA is already configured for your user.',
+    password_complexity_fail: 'Password does not meet the minimum complexity score of MIN_SCORE.',
+    invalid_trap: 'Invalid trap type.',
+
+    unable_to_grant_trust: 'Unable to grant trust. Grant token is invalid.',
+
+    invalid_un_or_pw: 'Invalid username or password.',
+    unable_to_complete: 'Unable to complete authentication: one or more errors occurred',
+}
diff --git a/locale/en_US/common.locale.js b/locale/en_US/common.locale.js
index df7e22a..7993850 100644
--- a/locale/en_US/common.locale.js
+++ b/locale/en_US/common.locale.js
@@ -4,4 +4,6 @@ module.exports = exports = {
     new_to_flitter: 'New to Flitter?',
     start_here: 'Start Here.',
     log_out: 'Log out',
+
+    invalid: 'Invalid',
 }
diff --git a/locale/en_US/iam.locale.js b/locale/en_US/iam.locale.js
new file mode 100644
index 0000000..12cf723
--- /dev/null
+++ b/locale/en_US/iam.locale.js
@@ -0,0 +1,4 @@
+module.exports = exports = {
+    policy_not_found: 'Policy not found with that ID.',
+    invalid_entity: 'Invalid entity_type. Must be one of:'
+}