There are similar strings in the JavaScript files, which aren't (yet) tagged for translation.  Are you comfortable searching through the JavaScript (here: https://github.com/edx/edx-ora2/tree/master/apps/openassessment/xblock/static/js/src)

For example, in the JS we update the status to "Saved but not submitted", which hasn't been changed in this PR:
https://github.com/edx/edx-ora2/blob/master/apps/openassessment/xblock/static/js/src/oa_response.js#L204