Changing the language in F-Droid isn't reflected until F-Droid is restarted.
When changing the language of F-Droid via settings, F-Droid used to correctly update the locale when leaving the preferences activity. Given the settings are now part of the main activity, there is not the same opportunity to update the locale. As such, F-Droid needs to be closed and re-opened before the language is updated. This should be fixed.
Note, this is similar to the issue discussed in #907 (closed) about how themes used to apply correctly. The code in that issue was also used to ensure the language was updated correctly.