by Dave Balch.
Hi Dave,
I think you want https://docs.moodle.org/27/en/Language_customization
You'll probably want to use Moodle's built-in strings editor, at Home ► Site administration ► Language ► Language customisation
...or - for a more hacky approach - you could edit both assignmentmail and assignmentmailhtml in mod/assign/lang/en/assign.php
HTH,
Dave.