I commented the question and asked:I'm afraid to ask, but don't you expect that changing of the language tag will somehow miraculously cause automatic translation of English tooltips into German?
AlwaysLearningNewStuff finally replied:
Yes! Isn't that what documentation for xml:lang and Language states?
If you set it to "de-DE" then Language changes from default English to German, then tooltips should change from English to German, right?
Of course not. It's not just "not", it's "of course not".
Please read my last comment where I talked about "gravitation demons". If you think that there are some demons inside a computer who translate any arbitrary text, tooltip or not, into one of the languages listed in the standard set of culture info, don't hold your breath. This thinking is in general counter productive and has nothing to do with your knowledge of WPF you are trying to refer to. Better look at any automatic translation tool. I'm less sure about translations between such close languages as English and German, but translation between some other languages is the infinite source of jokes. At the modern level of linguistic science and technology, this can only be a good translation assistant, never a translator…
Sorry for the inconvenience. :-)