In a mulitlanguage site you can't switch the language in the site tree. This should be possible.
I have a website with german, englisch and french.
The Site tree shows the german sitetree with german labels, etc.
If I give an account to my french friend or the translator, he will also see the german site tree. He's not able to switch the sitetree to another language. This should be possibe.
This is not the system language. A translator for en and fr may has french as system language but should be able to switch the language of the site trees.