[fenomas]

I don't think the writing matters a whit.

The only premise the story depends on is: that the camp guards saw the workshop and took it for granted that it was approved by somebody, since it was orderly and operating openly. If you accept that, it doesn't matter if the sign was amateurish or even upside-down - it would just look like something the workers had been told to make, or had made themselves to test the tools or to pass the time.

A bunch of posters here seem to be imagining that the sign was the lone keystone of the ruse, and that for some reason it needed to look like it was written by a native speaker or else the whole plan toppled. But nothing in TFA suggests that, it just says the sign was one of several things the POWs did to make the whole setup look like it had approval to be there.