It would be nice to have an idn category, so one could list all IDNs just by clicking the link. This would be dead simple to implement, just do a search for xn-- in your database.
Additionally, it would be nice to allow sellers to choose the language of an idn when they list it for sale. That way, you could also allow marketplace users to list e.g. all french idns.
You can view all IDNs in the marketplace by clicking on the "IDNs only" link in the "other options:" section.
Technically, the language of an IDN is already chosen once the name is registered. Our registration system looks for the correct language tag when registering an IDN.
Your suggestion on adding options to select a language while looking through a list of IDNs is a good idea, so I have added it to our feature request list. It'll probably be automatic though since we would already know the language tag of the IDN.