My oh my, how online translators suck. What I wanted to express was the following:
May we write in German in the insanity/madness forum? Perun, is my grammar right? I have forgotten so much ...
In other words, the online translator gets most words correct, but messes up the order of the words (leaves it pretty much unchanged from the German text, which it shouldn't).
I must make one reservation: I think I should have written "in der Wahnsinn-Forum" if Forum is feminine, and "in dem Wahnsinn-Forum" otherwise. I remembered it afterwards. If you go from one place to another, you use the accusative form of the article. In other words, cases where you would use "into" or "to" in English. If you would use "in", you use the dative form.