It's not so much being Facebook experts as it is actually having a solid pretext for an arrest.

I guess if Facebook isn't marking the automatic translations that is a problem, but they shouldn't be giving someone a bad day before they understand which part of the post he wrote and what it likely means.

I think it is 99% police. I do not use FB translate, but Google translate. Sometimes it works, sometimes it does not. I expect that it is not 100% reliable, and anybody who frequently uses the automated translation tools for critical tasks (presumably the police in this case) should know this. My suspicion is that this was just an excuse for the police to arrest somebody.