[Koha] How to search by Language on OPAC?

Hello, On the OPAC Advanced Search page one of the search options on the drop down list is "Language." Our collection contains a lot of foreign language films and it was my understanding we could use this field to search for all films in a particular language. Does anyone know the MARC tag that links to this search field? I've tried different ones, but have been unable to find the right one. Or maybe my understanding of the search field is incorrect... Thank you, Melisa Canales