As a source for a DropList, I would like to use a query that returns the list of languages defined for a specific item. Is it possible?
Not out of the box. Not in any way I could find at least.
Reason is; Sitecore Query (or even fast: query) works on item-level. I've never seen it used to query multiple versions of any single item, which is what you need here.
But with a little work
But there are options with a reasonably small code footprint. Alex Smagin (and others) wrote about, putting custom tokens into the datasource area of your field, to execute custom functionality.
Basically it involves hooking into Sitecore's
GetLookupSourceItems pipeline and injecting the specific dropdown list options you need there.