C# and Java allow almost any character in class names, method names, local variables, etc.. Is it bad practice to use non-ASCII characters, testing the boundaries of poor editors and analysis tools and making it difficult for some people to read, or is American arrogance the only argument against?
I would stick to english, simply because you usually never know who is working on that code, and because some third-party tools used in the build/testing/bugtracking progress may have problems. Typing äöüß on a Non-German Keyboard is simply a PITA, and I simply believe that anyone involved in software development should speak english, but maybe that's just my arrogance as a non-native-english speaker.
What you call "American arrogance" is not whether or not your program uses international variable names, it's when your program thinks "Währung" and "Wahrung" are the same words.