I realize that many films have actors speaking a variety of languages, but I would like to figure out how to filter my collection so that I could see which movies have the dialog almost entirely in French, for instance.

I don't want an audio channel with French dubbing or a French commentary track.

I figured there must be some way to go back and identify this stuff without creating tags for each language ....

But if you want to filter for all movies which have French as their original language you'll have to resort to tags. I tagged all my movies with a "Original Language" Tag with all the languages as subtags. This allows tracking unsupported languages and allows filtering like that.