(core) User language switcher

Summary:
New language selector on the Account page for logged-in users.
New icon for switching language for an anonymous user.

For anonymous users, language is stored in a cookie grist_user_locale.
Language is stored in user settings for authenticated users and takes
precedence over what is stored in the cookie.

Test Plan: New tests

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3766

app/common/UserAPI.ts

@@ -141,6 +141,8 @@ export interface UserOptions {
   // Whether user is a consultant. Consultant users can be added to sites
   // without being counted for billing. Defaults to false if unset.
   isConsultant?: boolean;
+  // Locale selected by the user. Defaults to 'en' if unset.
+  locale?: string;
 }
 
 export interface PermissionDelta {
@@ -331,6 +333,7 @@ export interface UserAPI {
   moveDoc(docId: string, workspaceId: number): Promise<void>;
   getUserProfile(): Promise<FullUser>;
   updateUserName(name: string): Promise<void>;
+  updateUserLocale(locale: string|null): Promise<void>;
   updateAllowGoogleLogin(allowGoogleLogin: boolean): Promise<void>;
   updateIsConsultant(userId: number, isConsultant: boolean): Promise<void>;
   getWorker(key: string): Promise<string>;
@@ -632,6 +635,13 @@ export class UserAPIImpl extends BaseAPI implements UserAPI {
     });
   }
 
+  public async updateUserLocale(locale: string|null): Promise<void> {
+    await this.request(`${this._url}/api/profile/user/locale`, {
+      method: 'POST',
+      body: JSON.stringify({locale})
+    });
+  }
+
   public async updateAllowGoogleLogin(allowGoogleLogin: boolean): Promise<void> {
     await this.request(`${this._url}/api/profile/allowGoogleLogin`, {
       method: 'POST',