RandomPHPDev,
Yes it's possible to change it. Click on Settings, and in the Settings menu to the left, go under General->Languages, pick the language you want to edit, and you can change almost all the terms used by the program. Softproof is amongst them.
I recommend you highlight the whole text, copy-paste it in a text editor so you can perform search and replaces, and then copy-paste it back in PSMW. This makes it easier that scrolling and manually searching for the words to change.
Hope that helps!
Regards,
Raphaël Lalonde Lefebvre