The point to have a thread is to talk to others, so a communal language is important, but not always possible. If you are writing in a language different from English I would suggest to write a google translation in the same message, so it's easier for everybody to follow what the message is about instead of skipping it. It is a matter of Netiquette. :)
Google provide an easy way to translate a page, however, it works well with one language, when you have 2 or 3 languages in the same thread it gets more complicated. Normally, when I see English, French and Spanish in the same thread I just give up and skip it altogether, which is not what the original creator probably intended when started the topic.
Russian (?) and Spanish are the only languages used in the flats which by logic should allow to create topics in those languages when referring to those couples.
However, most of us watching are neither Russians or Spanish, and most topics are created in English and follow ups one expect should be in the same language. However very often you see a topic starting in English going Spanish and French to go back to English again, which topic at that point has been already forgotten ;D.
By the way, RLC, I would personally love to see an Italian couple, as it is my native language. Let's swap from Spanish to Italian. I am sure you would find some nicer Italian couples ;)