Language files are automaticaly reset

Hello,

I have a very odd issue. I’ve a german WordPress running and replaced the language files to change the formal/informal translation (in german du/Sie).
For some reason the language files at wp-content/language are “reseted” after some time for some reason “automatically”. I already changed the CHMOD of the files to 444 so the file can only be read but even then the files get replaced.

The new files are used as soon as I upload the correct files again as I can see the text changing in the frontend.

I have no idea if it’s my hoster (1&1) giving me a backup once a day for the last week (pretty uncommon) or what is going on. As far as I know there is no plugin or function that is doing such a thing by default. And I’m not using any strange plugins. Just a slider, AJAX Login form for the blog etc. but nothing for translation or similar.
The WordPress language is set to german in the backend. Which sets the german language files in “Du” wording that I replaced with the “Sie” writing. (formal informal)

Any help is very much appreciated! Currently I can only go there and replace the files once a day again.

Thanks!
Felix