The change in https://gerrit.wikimedia.org/r/#/c/219168/14/repo/includes/api/SearchEntities.php broke the wbsearchentities API. Now it partly ignores the "language" parameter. The search is done in the language given via the parameter, but the returned labels and descriptions are always in the users current language set in it's wikidata.org cookie.
- Open https://www.wikidata.org/w/api.php?format=jsonfm&action=wbsearchentities&search=Meter&language=de
- Compare to https://www.wikidata.org/w/api.php?format=jsonfm&action=wbsearchentities&search=Meter&language=en, the label and descriptions do not change.
- Go to Wikidata.org and change your users language. Reload the two URLs above. The labels and descriptions change.
This bug is related to T98172: [Story] Implement Unit Selector widget because it relies on this API and it's ability to be localized.