How can I change the language to German?

Answer 1

If you mean keywords of the language like if, else, while, etc., you can't. You can still name your variables in German though.
If you mean the language of your IDE, see the site of the developers and check if they have a German version.

Answer 2

You can create a XML with all the keywords, then, send a cookie to the web browser after asking the user to input their language.