Hi @JanHumpal,
Logic for displaying Error messages on TranslationJob Tile resides in /libs/cq/gui/components/projects/admin/pod/dashboard/translationjobpod/body/body.jsp
Navigate to http://localhost:4502/libs/cq/i18n/translator.html -> Under Dictionaries -> /libs/wcm/core/i18n -> You can find these i18n keys.
On a very high level, this is displayed based on attribute named workflowOperationError which is set/available on Translation project's path which in turn should be set during Translation workflows (where the payload is our Translation project path)