[mod says] Welcome to Fluther! To answer your question: Using languages other than English to tag is against the rules. You could still technically do it, but your question would probably be sent back to you for editing. What's more, it just doesn't make sense if you consider how Fluther's tags work. I’m going to include an example from the guidelines to illustrate:
‘Tagging with synonyms – if you have a post about marijuana, is it helpful to tag it with “pot”, “grass”, “dope”? The short answer is “no”. We alias topics so matching will still work with any single topic. [...] When trying to pick which synonym to use please use the most popular or most recognized term (use your best judgment—“marijuana” or perhaps “cannabis” in this case).’
So, even using all the synonyms available in English usually doesn’t improve the system’s ability to match your question to older threads or users who might want to answer it. Synonyms from other languages wouldn’t have any followers or related questions at all.
Hope this help. Enjoy your Fluthering!