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.Isaac commented
The option to show the films from a selected country. Also would be nice an "Advanced Search" of some kind where you could filter by age, or decade for example and country, and not only by genre or actors/director.