I pasted SaboteurLauncher to the GOG version of The Saboteur and after that I activated the code for knife, but I don't think it's working. I tried using E and Z to do stealth kill (tapping and holding) but Sean was only doing standard stealth kills.
This knife should be activated in prologue or is it activated AFTER prologue (when we're coming back to France)?

Edit - holding E doesn't work. There is no stealth kill with nazi knife sadly.