Hey all,
This is a known bug that our Diablo 3 team is aware of and working to resolve. In the meantime the workaround that MissCheetah posted should work to set the game language.
Edit: I’m seeing reports that the issue should be resolved now actually. I just tested it and switching the language in the Blizzard app did correctly switch it in-game. Make sure the Blizzard app is updated and try re-testing it without the command line argument.