WP can be localized for iOS, Android and Windows Phone. However, there are many locales which have active teams which are no supported on some or all mobile OS. Because WP apps set their UI language based on the OS of the handset, the user is forced to accept whatever the OS language is which may not be their preferred language.
For example Scottish Gaelic (gd) is not a supported locale on iOS or Android. In the past, on Android one could use a Custom Locale app to 'fake' a locale but that was a) unwieldy for low-tech users and b) is no longer allowed on Android (4.2 onwards).
In order to support all locales properly, WP apps should allow the users to set the UI language manually, overriding the OS locale if they wish to do so. The initial WP locale can still be set according to the OS locale to cater for large, supported locales as long as a manual override is created.
I apologize for not being able to put forward a patch but I'm 99% localizer with no programming skills. Also sorry for the wordy bug report but if I wrote less, I would have to answer many questions later one (from experience on other projects)!