In the file .config/joplin-desktop/log.txt all messages (but one) are in English. That’s fine.
The only exception is the message: Synchronisation is already in progress. State: %s
which (probably by mistake) was translated into German and reads: Synchronisation ist bereits im Gange. Status: %s.
The translated message can be found in the actual de_DE.po file and should be corrected.
If it is in the de_DE.po file it means that the string was used in the code.
We can’t just remove it from the language file - it could be used somewhere else in the UI (and it would show up again, after re-creating the language files).
Instead, we have to remove the translatable string call _('...') from the logger code.