Lately I've been thinking more about tags and how the fact that users use them in different languages effect on the tagging system. In the case where I work (EU+education) we want to use tags in different languages as something that unifies people rather than divides them in different sections. That is why in our system we are NOT thinking of keeping multilingual tags and other annotations separated.
This type of separation along language and/or national lines can be seen in quite a few places on the Web. For example in Amazon, the reviews and ratings are not shared between the .com and .fr version. I understand that reviews and ratings, especially from experts and authoritative reviewers, are something really culturally biased, but I would think that it is interesting for readers in the US to know how a book has been received in France.
Lately in our team we've also talked about translating tags. Like if my tags, which I have added in English would be translated by someone, or a machine, into Finnish, French, etc. I don't like that idea. If I translate my tags, for example I add a tag in English and in Finnish, it's fine. But if it is done for me, I don't think it Ok. Let me explain:
I prefer to display to users only USER generated tags, not translations (neither people done nor automated ones). The key thing with tags, and the big difference compared to normal vocabularies, is that they not only describe the resource, but are associated to a user. This relation of users, tags and resources is fundamental!
In the scenario where tags are translated the following questions arises: to which user do you link a translated tag? In my opinion (say, if pushed to an edge), if a tag is translated, it ceases to be a tag and becomes just a mere keyword.
The above does not mean that tags could not have translations or equivalent terms in other languages or in its own language. Most likely in our system we will see lots of both. But the difference is that those tags all are created by other users and can be associated to users and resources. Translated keywords can be only associated to resources. This connection of users, tags and resources becomes our main asset for connecting people across the national and linguistic borders. Let's keep it that way!
If tags are translated, they could be used for other purposes than for displaying (I mean tag clouds, social navigation, etc). Translated tags could, for example, be helpful as keywords to make the search better. If a tag is translated, it should also be indicated in the metadata of the tags.