Change System Message on Mark Complete of Task (2013)

I need to change the system message that pops up when I set the Task to Complete to my custom message .Can you please advice if this can be done .I have had a look at the CrmTranslations file and this particular message does not appear to be there and I am struggling to locate where this is stored and then how I change it. We are using Dynamics 2013 On-Premise.

Regards