If you have access to the database, so the following update sql should do it:
"UPDATE lime_surveys_languagesettings SET surveyls_endtext="myNewEndMessage" WHERE surveyls_survey_id=[surveyID] and surveyls_language=[languageCode]"
This is what you see in the database description and in phpMyAdmin or a similar tool.
Joffm
Volunteers are not paid.
Not because they are worthless, but because they are priceless
Have you tried set_language_properties? This looks to take an array of properties you want to update and the end text is stored in the language settings table. I have not tested this, but its worth a look.
I tried this command as well, and it works perfects. Though I chose to finish my job with API for it's safer, thank you a lot! You've been greatly helper!