> 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 cannot know, but they could be ideological. For example, they had found it wonderful to use plain ASCII, no need for special keyboard layouts or something like that, and they decided that German would be much better without its non-ASCII characters. They could believe something like this, and they wouldn't say it aloud in the discussion with you because it is irrelevant for the discussion: you weren't trying to change German.