A long, long time ago, many Wikibase API messages were marked as ignored for translation in translatewiki (see T40675). Since then, the conditions changed: there is now a good way to translate API messages, so they shouldn't be ignored.
I already made a patch for this at https://gerrit.wikimedia.org/r/c/translatewiki/+/697903 , but I guess it needs review by current Wikibase developers.