Also here is a quote from Apple's document that might help:
To determine the language for your app, iOS considers not only the order of the user language preferences (in General > International > Language of the Settings application) but also the localizations your app declares it supports. Here is the detailed process:
iOS first looks up your user's most preferred language, which is the first entry in their language preferences.
It proceeds to check if that language is supported by your app. iOS searches your app bundle for an .lproj folder matching the preferred language. If the folder exists, iOS infers that your app has been localized for that language and chooses it for your app. Otherwise, iOS selects the next language in the user language preferences, then repeats the above check.
The dialect support in iOS may slightly change the above behavior. If your user's preferred language is a regional variant that is not supported by your app, iOS will try to fall back to a more generic language before giving up. For example, if your user's preferred language is British English and your app bundle doesn't contain an en-GB.lproj or en_GB.lproj folder, iOS then searches your bundle for an en.lproj folder and chooses English for your app if the folder exists.