I use PassAndroid (https://f-droid.org/packages/org.ligi.passandroid)

but it looks unmaintained now

I would imagine that this has similar root causes like Italian in South Tyrol. About 100 years ago in an attempt to forcefully italianize the german-speaking Tyroleans the fascists moved a lot of italians from all over Italy into South Tyrol, resulting in a very clean italian (somewhat “high-italian”) being spoken there, opposed to the various regional dialects all over Italy. The clean language is a more common ground between everyone, so it makes sense to default to that (and is a lot closer to the language foreigners learn)