Apparently ParserOptions::getUser()->getOption( 'language' ) doesn't provide the right language here, only ParserOptions::getUserLang() does. We use the former as an optimization in Scribunto_LuaWikibaseLibrary::getLanguage and Scribunto_LuaWikibaseEntityLibrary::getLanguage to only split the parser cache when actually needed.
As far as I see, we only call getLanguage when a Lua function is called anyway, so this hack probably doesn't save us much, so I suggest dropping it.
Change 305439 had a related patch set uploaded (by Hoo man):
Don't abuse ParserOptions::getUser()->getOption( 'language' ) to get the user lang
Change 305439 merged by jenkins-bot:
Don't abuse ParserOptions::getUser()->getOption( 'language' ) to get the user lang
It is still not working right. I just set my preferred language to Polish and executed lua code with
both returned "en" instead of "pl". At the same time call to message "{{int:lang}}" without LUA returns correctly "pl".
mw.message.* is not part of Wikibase, thus not affected by this. The changes mentioned here only affect the functionality to access data from the Wikibase repository (Wikidata) (mw.wikibase.*).
