Actually, the api returns only english error messages (code and info), see: https://github.com/wikimedia/mediawiki/blob/e369f66d0029594d1ce289f7a505d3ce20250828/includes/api/ApiBase.php#L1344
At least, the info message should be localised to the user language. Usage example: MobileFrontend displays the message in the info field for some error codes (https://github.com/wikimedia/mediawiki-extensions-MobileFrontend/blob/master/javascripts/modules/editor/EditorOverlay.js#L341).