Thanks for your help boughtonp. It's good to know that it's part of the source rather than a mere menu item that I can edit (meaning I can stop wasting time looking for such a menu item). Someone on Reddit (where I had also posted this) suggested that the file
src/wmtaskbar.cc had the tag I wished to edit. So, via the instructions
here, I got the source for the Sid package, edited the
wmtaskbar.cc file, and built it for my Debian stable system (thus both fixing the spelling for my localization and also upgrading the package from icewm 1.4.3.0~pre-20 to icewm 1.9.2). So, it now has been changed.