I haven't found the text for DT_CUT_OFF_DATE_MESSAGE ("We are no longer accepting registrations for this event. Thank you. ") in the language files to translate.
The Cut-off date message does not come from the language files. It is a configurable message in the backend Configuration page of DT Register. So the message is stored in the database. If you are doing a language translation, that would be done with our joomfish content element files.