No support for ISO 639-2 languages
Since Android 5.0 and up it's possible to use ISO 639-2 languages but F-droid lacks of it. In the wiki there is at least one language called Asturian which uses this regulation.
I guess the auto-translations-import script from the wiki doesn't work with 3 letter code languages and that's a problem. I provide you links to bugs in AOSP about this question:
https://code.google.com/p/android/issues/detail?id=63358
https://android-review.googlesource.com/#/c/82116/