With regards to metadata I always consider it as "data about data", hence I wouldn't use it directly for content purposes, the only exception to this for me is metadata on a multimedia component (since you don't have regular content fields there). But then looking at keywords I think this could at some point be the other exception to my rule.
If you build up your navigation based on keywords, there is a use case for additional data, including (translatable) content. The metadata of keywords is translatable through translation manager, but it is not sent forward to the broker if I recall correctly. So to use it, you would have to store it as a sitemap page for instance (writing the keyword metadata through a template/C# TBB in a page).