refactor(fxa-settings): Change default locale of fxa-settings from en-US to en

Because:

- We want to standardize the default locale of all packages to en.

This commit:

- Rename all fxa-settings en-US.ftl files to en.ftl
- Change destination path in merge-ftl task to locales/en

Closes #FXA-6003

packages/fxa-settings/Gruntfile.js

@@ -10,8 +10,7 @@ module.exports = function (grunt) {
     concat: {
       ftl: {
         src: ['.license.header', 'src/**/*.ftl'],
-        // TODO: change dest to `en` in FXA-6003
-        dest: 'public/locales/en-US/settings.ftl',
+        dest: 'public/locales/en/settings.ftl',
       },
 
       // We need this for tests because we pull the latest from `fxa-content-server-l10n`

packages/fxa-settings/src/models/AppContext.ts

@@ -92,7 +92,7 @@ export function initializeAppContext() {
     config,
     account,
     alertBarInfo,
-    navigatorLanguages: navigator.languages || ['en-US', 'en'],
+    navigatorLanguages: navigator.languages || ['en'],
   } as AppContextValue;
 }