I believe it would be quite useful to be able to change the application language in the preferences dialog.
For example, my system language is German because I'm trying to learn it, but I like to use some applications in English because otherwise I have no clue of what they are saying.

I could work on this if you like.
From what I saw, DB Browser would probably need to be restarted to apply the change (as does Qt Creator when changing the language).

One problem would be where to put this option in the preference dialog. I thought about creating another tab (e.g. "Interface"), but I'm not sure if a tab with only a single drop down menu would look nice.