> I just never understood why I, as the non-German, was forever the one trying to convince them that Germans would probably prefer to use their software in German...

I've heard that German is often one of the first localizations of (desktop) software because there were often super-long words in the translations of various concepts, so if you wanted to test typeface rendering and menu breakage it was good language to run through your QA for that.

Or you use pseudo-localization, which does simple programmatic substitution to make all English strings longer by e.g. doubling letters or inserting random non-alphabetic characters, adding diacritics etc while still retaining readability to English speakers.

Windows actually ships with a locale like that.