Plesk is configured with non-English default language. But it gets switched to English after visiting Tools & Settings > Backup Manager > Scheduled Backup Setting. The language gets changed back to the configured non-English after re-login to Plesk.
This is Plesk internal issue #PPPM-1738, which has been fixed in Plesk 12.0 microupdate #9.
Apply the latest Plesk microupdates.