Medium formats are translated on RG pages and in the top right section on release pages, but untranslated in the header of the release pages (maybe because it says Medium 1, etc. there, and not just Medium?) and in release editor.
Cf. e.g. http://beta.musicbrainz.org/release-group/2682d0ba-b258-3485-9692-287922b8b264 and any of the releases there.