Here is a good question that comes up quite a bit in discussion.
How does FAST/Sharepoint handle the language refiner?
There are three language fields in FS4SP, DetectedLanguage, language, and languages. There is a description field foreach field but SP does not give much more information to know what is really
going on behind the scenes here. Generally there is a crawled property to managed property mapping that helps determine how a field is populated however
there is no crawled property mapping for the language and languages fields.
So you need to understand that “language” is an internal property that holds the primary language that got detected for the document, at feeding time, as an ISO
639-1 code (en/fr/de/etc…) or “unknown”.