The /ws/2/ json output generally uses english plurals for keys of lists, so for example [1] has a list of release-groups. The search server does use an english plural for releases, but not for release-groups in [2].

[1] http://musicbrainz.org/ws/2/release-group?artist=1f245b43-f9ba-4d02-a70b-cd08a99cd1c9&fmt=json

[2] http://musicbrainz.org/ws/2/release-group?query=arid:1f245b43-f9ba-4d02-a70b-cd08a99cd1c9&fmt=json