I have now investigated the issue further.
The issue seems to be the Aurora Store and affects Apps with split APK that contain a German locale.
(The issue might exist as well in relation to other locales.)
Since version 4.7.2 (4.7.1 ?) Aurora often downloads the English locale regardless of the language spoof setting, the app language setting or the system language setting.
This happens when Aurora isn’t displaying the app information in German but instead in English or another language.
Unfortunately this happens randomly since version 4.7.2 (4.7.1 ?) and is quite annoying. Sometimes you are lucky and Aurora displays the app information in German, then it will also download the German locale and your app will be accordingly in German.
If you update your app, however, at a time when Aurora displays the app information in another language, your app language will change again with the update.
Version 4.7.0 of Aurora Store doesn’t have this issue yet.
Is there a way to downgrade the Aurora Store to version 4.7.0 on iodéOS without creating trouble?
(Several people reported problems during the last weeks here on the forum when uninstalling/reinstalling Aurora Store trying to fix the current Aurora issues on FP5.)