Some quick googling has revealed the following interesting links to sites that have taken on the multilingual challenge.

Xiha Life - www.xihalife.com

"Unlike other social networks that make users choose one language for all communications, XIHA Life users can easily read and write in multiple languages, allowing them to connect with family, friends, and colleagues anywhere in the world in the most appropriate language."
- www.sociableblog.com

Faviki and Multilingual Semantic Tagging

"Semantic tagging is not limited to English language. Faviki lets you tag in 14 different languages by suggesting tags from DBpedia. All popular world languages are supported: Chinese, Japanese, French, German, Spanish, Portuguese, Italian, Russian."
- www.faviki.com

VERY INTERESTING!

Common Tag Web Standard

"Common Tag is an open tagging format developed to make content more connected, discoverable and engaging. Unlike free-text tags, Common Tags are references to unique, well-defined concepts, complete with metadata and their own URLs. With Common Tag, site owners can more easily create topic hubs, cross-promote their content, and enrich their pages with free data, images and widgets. "

"The Common Tag structural model is very simple. It states that a piece of content addressable through a URL (a "resource") can be "tagged" with one or more Tag structures. Each Tag can contain a pointer to another resource that identifies the concepts described by the content, unambiguously indicating what the content "means." Optionally, the Tag may also contain information about when the Tag was created (the "tagging date") and what human readable "label" should be used when listing the concepts covered in the content. "
- http://www.commontag.org/

On my site I have three working languages: Estonian, Russian and English. My guess is that the Common Tag approach would provide a central "tagbase" to which user's tags could be paired according to semantic concept.