Set lang in api createaccount regardless of $wgLoginLanguageSelector

$wgLoginLanguageSelector is related to how the interface of
special:userlogin works. It doesn't make sense to listen to it
in the api module

Also throw error if invalid lang code is specified (for sanity)

Change-Id: Ibf72fd5a318cadc450b3b579757ee8145864a27a
Merge "Set lang in api createaccount regardless of $wgLoginLanguageSelector"