I'm not sure, but I've always assumed
Language means the original language, and
Translation is the language is has been dubbed to—in that stream. (Note that audio information is recorded separately for each stream, and these are single value fields.) But I can't say I've ever seen it written that way in a file's tags. It seems the language of the stream is always in
Language, whether it's dubbed or not.
Translation is more often something completely unrelated to language (e.g., it's a Google copyright notice for trailers I've downloaded from YouTube).
Nostra posted as I type. I suppose his answer is consistent with mine.
Language is going to be the language of the stream, not the original language. So I suppose the sensible way to indicate
Translation is "from [original language]." The collapsed stream info will then read, "[dubbed language] - from [original language]."
BTW, this field type shows a list of all values existing in the database in the combo box. The list of values is can be edited in
Preferences>Lists.