I think that a merge makes sense in topic areas that concern everyone (like "Upgrade Warnings" and "Announcements"). On the other hand, German and English subforums for "Errors in the manual" could be useful (as soon as the English manual will be available) and other topics that could concern only certain language communities.
My proposal concerning the question about mixing languages inside a thread (as mentioned by der_bud):
Inside the merged topic areas we could motivate our users to write the subject name in English. In case they do not have the necessary English skills, they could write the post in their respective native language and add an English translation generated by some translation tool.
The following answers can then be written in (maybe simple) English. I guess we can expect at least some passive knowledge of English nowadays, especially among users of a rolling release Linux distro.
der_bud's point about the quality of online translators today is also interesting. Such mixed language conversations are quite common in Scandinavia for example, as speakers of Danish, Swedish, and Norwegian can understand each other quite well. In our case this probably depends on the willingness of non-German speakers to use such tools.
If siduction becomes more popular and, for example, the German and English communities become big enough to organize themselves, a further separation in the forum can be useful. At the moment, we have maybe a couple of posts a day. So the many subforums are more confusing than helpful right now.