Offcourse it's allowed, as long as you know that language and have what to say. Here on Bitcointalk, nobody knows where you're from, so there is no way to know that this board doesn't belong to you.
In a sense, it’s similar (not exact) to what goes on in my Spanish local board. There are (a few) people there from Spain, Mexico, Venezuela and so forth, and we often participate in threads that have a country-based content without any issue (doing a bit of prior background topic checking may help).
I think Spanish is a bit different thing. Spanish language, same like French is global language at some level. It can't be compared with English, but it's also used in many countries. These boards are dedicated to language, rather than specific country, like Dutch, Philippnes or German.