Please note that the version of the extension in Subversion is experimental and not necessarily complete
So I should rather use the original one by Daniel Arnold? (http://www.mediawiki.org/wiki/Extension:Multilang)
it may have been broken, or there may be issues with it which haven't been picked up in brief testing, including security vulnerabilities.
Well, if you did brief testing, that means it worked for you at some point!? But I don't see anything at all at the moment... Security vulnerabilites are not a huge issue at the moment.
Cheers,
Frederik