diff --git a/.gitignore b/.gitignore
index e9931c6b..8b622f6a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -7,6 +7,8 @@
 /static/*.bundle.js.*.txt
 /grist-sessions.db
 /landing.db
+/docs/
+/sandbox_venv*
 
 # Build helper files.
 /.build*
diff --git a/README.md b/README.md
index 24bafba3..0a8e6be9 100644
--- a/README.md
+++ b/README.md
@@ -207,6 +207,7 @@ GRIST_BACKUP_DELAY_SECS | wait this long after a doc change before making a back
 GRIST_DATA_DIR      | directory in which to store document caches.
 GRIST_DEFAULT_EMAIL | if set, login as this user if no other credentials presented
 GRIST_DEFAULT_PRODUCT  | if set, this controls enabled features and limits of new sites. See names of PRODUCTS in Product.ts.
+GRIST_DEFAULT_LOCALE  | Locale to use as fallback when Grist cannot honour the browser locale.
 GRIST_DOMAIN        | in hosted Grist, Grist is served from subdomains of this domain.  Defaults to "getgrist.com".
 GRIST_EXPERIMENTAL_PLUGINS | enables experimental plugins
 GRIST_HIDE_UI_ELEMENTS | comma-separated list of parts of the UI to hide. Allowed names of parts: `helpCenter,billing,templates,multiSite,multiAccounts`
diff --git a/app/client/models/ColumnToMap.ts b/app/client/models/ColumnToMap.ts
index a086f526..48638e53 100644
--- a/app/client/models/ColumnToMap.ts
+++ b/app/client/models/ColumnToMap.ts
@@ -9,6 +9,8 @@ export class ColumnToMapImpl implements Required<ColumnToMap> {
   public name: string;
   // Label to show instead of the name.
   public title: string;
+  // Human description of the column.
+  public description: string;
   // If column is optional (used only on the UI).
   public optional: boolean;
   // Type of the column that widget expects.
@@ -20,6 +22,7 @@ export class ColumnToMapImpl implements Required<ColumnToMap> {
   constructor(def: string|ColumnToMap) {
     this.name = typeof def === 'string' ? def : def.name;
     this.title = typeof def === 'string' ? def : (def.title ?? def.name);
+    this.description = typeof def === 'string' ? '' : (def.description ?? '');
     this.optional = typeof def === 'string' ? false : (def.optional ?? false);
     this.type = typeof def === 'string' ? 'Any' : (def.type ?? 'Any');
     this.typeDesc = String(UserType.typeDefs[this.type]?.label ?? "any").toLowerCase();
diff --git a/app/client/ui/CustomSectionConfig.ts b/app/client/ui/CustomSectionConfig.ts
index c36c60dd..f8e4cd65 100644
--- a/app/client/ui/CustomSectionConfig.ts
+++ b/app/client/ui/CustomSectionConfig.ts
@@ -4,7 +4,7 @@ import * as kf from 'app/client/lib/koForm';
 import {ColumnToMapImpl} from 'app/client/models/ColumnToMap';
 import {ColumnRec, ViewSectionRec} from 'app/client/models/DocModel';
 import {reportError} from 'app/client/models/errors';
-import {cssLabel, cssRow, cssSeparator} from 'app/client/ui/RightPanelStyles';
+import {cssHelp, cssLabel, cssRow, cssSeparator} from 'app/client/ui/RightPanelStyles';
 import {cssDragRow, cssFieldEntry, cssFieldLabel} from 'app/client/ui/VisibleFieldsConfig';
 import {basicButton, primaryButton, textButton} from 'app/client/ui2018/buttons';
 import {colors, vars} from 'app/client/ui2018/cssVars';
@@ -61,6 +61,10 @@ class ColumnPicker extends Disposable {
         this._column.optional ? cssSubLabel(" (optional)") : null,
         testId('label-for-' + this._column.name),
       ),
+      this._column.description ? cssHelp(
+        this._column.description,
+        testId('help-for-' + this._column.name),
+      ) : null,
       cssRow(
         select(
           properValue,
diff --git a/app/client/ui/PageWidgetPicker.ts b/app/client/ui/PageWidgetPicker.ts
index 92618bf4..bc58ef3f 100644
--- a/app/client/ui/PageWidgetPicker.ts
+++ b/app/client/ui/PageWidgetPicker.ts
@@ -469,11 +469,13 @@ const cssEntry = styled('div', `
   align-items: center;
   white-space: nowrap;
   overflow: hidden;
+  cursor: pointer;
   &-selected {
     background-color: ${colors.mediumGrey};
   }
   &-disabled {
     color: ${colors.mediumGrey};
+    cursor: default;
   }
   &-disabled&-selected {
     background-color: inherit;
diff --git a/app/client/ui/RightPanelStyles.ts b/app/client/ui/RightPanelStyles.ts
index bbd94022..184e8ff1 100644
--- a/app/client/ui/RightPanelStyles.ts
+++ b/app/client/ui/RightPanelStyles.ts
@@ -13,6 +13,12 @@ export const cssLabel = styled('div', `
   font-size: ${vars.xsmallFontSize};
 `);
 
+export const cssHelp = styled('div', `
+  margin: -8px 16px 12px 16px;
+  font-style: italic;
+  font-size: ${vars.xsmallFontSize};
+`);
+
 export const cssRow = styled('div', `
   display: flex;
   margin: 8px 16px;
diff --git a/app/plugin/CustomSectionAPI.ts b/app/plugin/CustomSectionAPI.ts
index 51bb7873..2d1b68b9 100644
--- a/app/plugin/CustomSectionAPI.ts
+++ b/app/plugin/CustomSectionAPI.ts
@@ -11,6 +11,10 @@ export interface ColumnToMap {
    * Title or short description of a column (used as a label in section mapping).
    */
   title?: string|null,
+  /**
+   * Optional long description of a column (used as a help text in section mapping).
+   */
+  description?: string|null,
   /**
    * Column type, by default ANY.
    */
diff --git a/app/server/lib/ActiveDoc.ts b/app/server/lib/ActiveDoc.ts
index 2b8d968a..b0889f4b 100644
--- a/app/server/lib/ActiveDoc.ts
+++ b/app/server/lib/ActiveDoc.ts
@@ -129,7 +129,7 @@ bluebird.promisifyAll(tmp);
 const MAX_RECENT_ACTIONS = 100;
 
 const DEFAULT_TIMEZONE = (process.versions as any).electron ? moment.tz.guess() : "UTC";
-const DEFAULT_LOCALE = "en-US";
+const DEFAULT_LOCALE = process.env.GRIST_DEFAULT_LOCALE || "en-US";
 
 // Number of seconds an ActiveDoc is retained without any clients.
 // In dev environment, it is convenient to keep this low for quick tests.
diff --git a/sandbox/grist/functions/date.py b/sandbox/grist/functions/date.py
index c38651d7..5a24340a 100644
--- a/sandbox/grist/functions/date.py
+++ b/sandbox/grist/functions/date.py
@@ -292,7 +292,7 @@ def DATEVALUE(date_string, tz=None):
   >>> DATEVALUE("asdf")
   Traceback (most recent call last):
   ...
-  ValueError: Unknown string format
+  dateutil.parser._parser.ParserError: Unknown string format: asdf
   """
   return dateutil.parser.parse(date_string).replace(tzinfo=_get_tzinfo(tz))
 
diff --git a/sandbox/requirements3.txt b/sandbox/requirements3.txt
index 38fb0a18..f0e39250 100644
--- a/sandbox/requirements3.txt
+++ b/sandbox/requirements3.txt
@@ -13,7 +13,7 @@ backports.functools-lru-cache==1.6.4
 chardet==4.0.0
 enum34==1.1.10
 et-xmlfile==1.0.1
-html5lib==0.999999999
+html5lib==1.1
 iso8601==0.1.12
 jdcal==1.4.1
 json_table_schema==0.2.1
@@ -21,12 +21,12 @@ lazy_object_proxy==1.6.0
 lxml==4.6.3                # used in csv plugin only?
 messytables==0.15.2
 openpyxl==2.6.4
-python_dateutil==2.6.0
+python_dateutil==2.8.2
 python_magic==0.4.12
 roman==2.0.0
 singledispatch==3.6.2
 six==1.16.0
-sortedcontainers==1.5.7
+sortedcontainers==2.4.0
 webencodings==0.5
 wrapt==1.12.1
 xlrd==1.2.0