I did some testing during customer engagement with having a multilingual SharePoint 2010 termstore. What I wanted to know is when translateds labels are being used. Here is the answer:
Within the termstore I configured the folowing term:
Within the site settings / language settings of the site I have added the language "German" and then I am able to switch the site to German language via the user's menu:
After switching the language to "German" the terms will now be shown in German language:
If there is no label for the selected language available the default label will be used. Within the next screenshot this is the case for the term "Four" since I did not apply a German label for that term in the termstore:
Furthermore the autocomplete functionality when applying terms will also take care of the configured language:
And also for missing lanugage specific labels:
Will check soon it will work the same way in SP2013 and update this article.