In reply to Caspar Hübinger.

That is the only concept for me that makes sense. The general concepts are blog post in 1 language or having multiple languages and have one post translated to another language (not meaning always the same posts).
So "language-per-post" targets the first. It targets the users who sometimes posts something in another language. Adding a selectbox in an area what is already crowded doesn't make to much sense to me.