That already exists, more or less
We now have support for browsing by country and language. As some films have multiple languages listed, browsing French-language films (https://letterboxd.com/films/language/french/ as an example) will show English-language films as well (where those films have portions of dialog in French). We’d like to improve this in the future to be able to show films whose principal language is French.
I know for a fact that TMDb has this info, and it'd be cool to search for movies based on languages they're filmed in/otherwise contain.
I understand that most of the data for movies is taken off of TMDb, but can IMDb or another database with such a feature be used? Even if a movie has several countries that produced it, I'd still like to know