Speak another language other than English on the forums?

Depends on what you say. For the main boards though, it is quite preferable that you speak English, especially if we know you can.

I tend to warn people who do not speak English on the market boards because it causes confusion in haggle wars.
Private conversations can be done via PM, if you really MUST speak other languages at least translate what you are saying into English in the same post.

Ok I understand thx