The REST API's error response schema includes an object (messageTranslations), for returning key-value pairs of language code to translated error message.
The included languages, defined in EntryPoint::getTextFormatters are the content language and English. The messageTranslations object is made in ResponseFactory::formatMessage, using these languages.
It would be helpful to add the user language too, if it differs from either of these. Tagging Platform Engineering in case there's a particular reason why not to include the user language.