Fieldname rather than translation for language of materials appearing in the faceting sidebar

Looks like this facet was introduced with https://archivesspace.atlassian.net/browse/ANW-1018 and included a translation for the frontend en.yml that has somehow gone missing. (Looks like may have been overwritten by browse/search prefs work.) Will add langcode: Language of Materials back to frontend en.yml and the other three once some pending changes have been merged.