OK, maybe I'm just a pessimist, but some real reasons (if you understand the language well enough) is that English is usually the original and so the localization can be ok or bad, but it's usually not better than the original. And as I said, this is German and not some language spoken by just a few people.
That said, I'd love to know more about the generic impression of how well localization works for Spanish or Russian, languages with a lot more speakers. I suppose with UTF-8 it's at least only a few mojibake errors and not total mayhem like in the ASCII times...
And no, I'm not arguing other languages should not be on the internet but for technical stuff (and news) I somehow really prefer English, maybe it's because I've been hanging around online and in chatrooms in English for too many decades.