Considering that the site is about Language Learning, studies is inferred.
Should we use the tag at all?
studies, to me, means "studies, published in a peer reviewed journal". Not "I'm doing my language studies before bed."
Perhaps a less ambiguous tag can be used for this. I like @Gille's suggestion of reference-request. It doesn't have the ambiguity shortcomings of research (is the question asking about results of past research, or how to conduct research, etc)?
No, I don't think it's necessary. As you said, I think studying is implied given the context of the site.
I don't like the word "studies." It is basically implied by the site, that we are "studying" languages.
A better usage might be something like "field," or "discipline." That would imply some "subset" of language learning.
Peer-reviewed-evidence might be a more precise tag for the same purpose.