It's not ideal, but our approach has been to use primarily 'English' tags, unless a particular form language tag shows up a lot in search results. We use Tags by Solspace and it's great, but what we found is just as @Think Graphical mentioned, "the tags get cluttered very quickly, with all the multiple languages".
Our system, maybe different from others, so obviously, evaluate against what it is you're trying to accomplish. Our client basically produces multilingual publications on various issues concerning consumers— from credit to banking to identity theft, etc.
Frequently, the person searching for the publication in a particular language, doesn't actually speak that language or maybe can't read well in the language. For example, a social worker helping a first generation immigrant from Korea or the grandchild of a Hmong immigrant. The grandchild speaks English and Hmong, but may not read the language at all.
We found, when testing, that english language tags got more use than the foreign language tags, but search was a different matter. Since we were trying to limit the number of tags per entry (reduce clutter, improve/keep SEO ratings), removing tags duplicated in multiple languages was the first thing to go.
For us, search is fully multilingual compatible, and that's what we encourage for looking for multilingual words and phrases.