Can I suggest to put a credits line when someone copies from the site?
I've seen it implemented on some site (i'll try to find it again). The basic idea is, if i copy a translation from the site, a Credits line will be automatically added at the end of copied translation.
The credits line can contain the username and/or name of the contributor and also a link to that particular translation.
I know that this won't prevent them from removing the credits once they have pasted elsewhere but i thought it would help.