Can't talk for everyone but I always use English in all my code for everything (Variables, Database, Filenames etc.) Because it's a real pain to be consistent otherwise. In the beginning I used to write parts of my code in German because I thought it would be easier to understand but it just made my life harder in the long run because I had to deal with inconsistencies all the time.

And after my initial trouble with some special characters like the French accents (é, è, à etc.) and the German umlauts (ä, ö, ü) I made myself do everything in ASCII.

General rule is: Code and back-end stuff 100% English and only basic ASCII characters. Front-end in UTF-8/LAT1 but when possible avoid placing special chars in code (example ü in HTML only as ü not as ü)