Given an entity ID and a language code (for the T227759 use case, the page language), it returns the label, including its (BCP-47) language code (i. e., the language, not the for-language, of the wbgetentities response), while applying language fallbacks.

The directionality is determined elsewhere, most likely from $.uls.data.getDir().

This is most likely implemented using action=wbgetentities&props=labels&languages=___&languagefallback=1&ids=___.

Return value could be a FingerprintableEntity or something else; to be decided.