If you're concerned about someone switching to German in following posts, you might consider writing a few sentences in English, and using Danke as the start or end to your post.
I agree with Daen that this is a great way to dissuade them from trying to start a German conversation with you.
Through the years, Iíve picked up greetings and thanks in a few languages (I only speak English, but have studied quite a few languages superficially). German happens to be one, since my family has German roots. There are times when "danke", "domo", or "gracias" come out of my mouth without even thinking.
I try not to make assumptions, but I think that when you have enough exposure to a language, it's inevitable that certain words will become part of your lexicon, and itíd be almost strange to me that on a bilingual board (or offline group, wherever) some of the simpler phrases of one language don't start being used by non-fluent members.
Ed. b/c spelling "speak" as "speek" doesn't support my claiming to speak English