I think this is an important feature. The collection and metadata features of Trakt are far ahead of what other websites can offer, however it is very surprising that the language of the movie or show cannot be set. To me it is a very important use case to filter the view of my collection to only include shows in English or, for example, Korean.

Alternatively to adding audio language and subtitle language fields, I could see user defined fields as incredibly helpful. This is definitely what I would wish to see long term, but I understand that this might be difficult to implement in the short term, so I see a strong need for the audio fields being included in the metadata.