Jürgen is definitely German but he is likely to answer your questions in English based on his previous policy, z.B. his remark "ohne Englisch geht im Computerbereich nichts"!
All languages are allowed on this forum. However you may frequently receive a reply in English if the person helping you doesn't speak your language well, or simply prefers to answer in English. The forum administrators (of whom I used to be one but am no longer one) have encouraged people to feel comfortable posting their questions in their native languages.