Locales: Performed cleanup and alignment of locale handling

- Reduced app settings down to what's required.
- Used new view-shared $locale object instead of using globals via
  config.
- Aligned language used to default on "locale" instead of mixing
  locale/language.

For #4501

app/Users/Models/User.php

@@ -12,7 +12,7 @@ use BookStack\Api\ApiToken;
 use BookStack\App\Model;
 use BookStack\App\Sluggable;
 use BookStack\Entities\Tools\SlugGenerator;
-use BookStack\Translation\LanguageManager;
+use BookStack\Translation\LocaleManager;
 use BookStack\Uploads\Image;
 use Carbon\Carbon;
 use Exception;
@@ -346,7 +346,7 @@ class User extends Model implements AuthenticatableContract, CanResetPasswordCon
      */
     public function getLanguage(): string
     {
-        return app()->make(LanguageManager::class)->getLanguageForUser($this);
+        return app()->make(LocaleManager::class)->getForUser($this)->appLocale();
     }
 
     /**