That looks like a bug, is supposed to display:
“This post requires staff attention because I believe it contains content that is illegal.”
This appears in that system dialogue box now when I have tested this, but e-mail notification shows similar to what you see there:
Translation missing: en_GB.flag_reasons.illegal
(That is with main update branch not beta or tests-passed)
Same with stable branch for message/e-mail notification:
Translation missing: en.flag_reasons.illegal
That is only sent if response to flag is to hide post and give author option to edit.