LocaleBundle | Locale Bundle for symfony2 | Web Framework library

The user's setting for preferred language (as set in System Preferences) is stored in user defaults under the key "AppleLanguages". It's an array of language identifier strings (like "pt-BR").

The user's setting is in the global domain but, as with all user defaults, you can override that in more local domains. In particular, you can set it in the application domain to affect just your app.

So, I think you should just store an array with the identifier for the language chosen by the user for your app under the "AppleLanguages" key and then let the system do its thing. You should not try to reproduce the resource search algorithm yourself.

If you want to keep using your technique, you can use the localization properties and methods of Bundle to pick one of the actual localizations available in a bundle according to a list of preferred ones. That is, don't query the localizations and then check if it contains your locale identifier; use preferredLocalizations(from:forPreferences:) to do the search for you. It likely uses a more sophisticated algorithm. Also, keep in mind the distinction between locale and language and, especially, locale IDs vs language IDs. The represent different things and have different forms, but you seem to be using them interchangeably.