Set up localization with feature flag control (#9649)

Refers #8128 

Changes Introduced:
- Added i18n configuration.
- Added a feature flag for localization.
- Enabled language switching based on the flag.

---------

Co-authored-by: Félix Malfait <felix@twenty.com>

packages/twenty-server/src/engine/core-modules/feature-flag/enums/feature-flag-key.enum.ts

@@ -15,4 +15,5 @@ export enum FeatureFlagKey {
   IsCommandMenuV2Enabled = 'IS_COMMAND_MENU_V2_ENABLED',
   IsCrmMigrationEnabled = 'IS_CRM_MIGRATION_ENABLED',
   IsJsonFilterEnabled = 'IS_JSON_FILTER_ENABLED',
+  IsLocalizationEnabled = 'IS_LOCALIZATION_ENABLED',
 }