I'm not speaking for the moderators. The following is solely and simply my point of view:
I do not see what's so complicated or difficult to see about the lock. It's always nice when mods enter a note as to the reasons for locks. Nevertheless, we do know that AT mods don't always reveal reasons for locks. Sometimes the reason seems obvious or you're expected to know why.
In my view, there are several reasons and I do not believe they are hard to see:
1) This forum isn't geared toward the German audience.
2) German is not a universal language.
3) It makes it difficult for the moderators to do their jobs if they do not comprehend what is written or spoken and need an interpreter or translator often. Anything could pass therefore. People can easily nef too.
4) Nothing forbids any communication in foreign languages. What is understood, though, is that you don't deliberately and continuously communicate in a foreign language. There have been past threads with foreign languages spoken, but the intent and context was what mattered. Most weren't explicit threads with titles and contents in the foreign language of choice. It wasn't done to make exclusions. It was done because the parties involved needed help or only posted that as an occasional tease or joke. In this case, it seemed apparent, to me at least, that the individuals involved did this with intent to exclude or cause discomfort.
5) Imagine if other parties decided to start all French, Slavic, Korean. etc. threads. I'm sure it would get uncomfortable for you too. I recall you were one of the unfaultering critics of Wduaqnug's broken English. Apply the same concept to people not comprehending your language of choice. Don't forget that this is a business and image counts. This forum, like any other, has an image to sell and uphold.
Again, this is my expressed view. I am not by any means speaking on behalf of the moderators.