nail down locale selection process for localized app data

%0.103 - UX Overhaul adds support for localized text and graphics for each app. android-24 expands the single language preference into a language priority list. F-Droid has the additional language chooser preference. We need to hear more form users about how to best put these pieces all together, then we can hone in on improving the policy that was implemented in !496 (merged).

Here is an overview of the Android locale selection logic: https://developer.android.com/guide/topics/resources/multilingual-support.html#postN

Part of this also includes the question about whether content from multiple locales should be mixed on the same screen. That approach would ensure that text/graphics fields are more likely to be filled in, but it might often produce pages that look sloppy since they have mixed spellings (color vs colour), mixed characters (Simplified vs Traditional Chinese), mixed alphabets (Uzbek/Kazakh/etc in Cyrillic and Latin letters), mixed grammar styles (de-CH vs de-DE), etc.

mentioned in merge request !496 (merged)

added first-timer label