I actually don't think the language is necessarily problematic because it's Jack (canon gives us plenty of time to see that Jack was a complete asshole pre-1999), although the characterization issue is, of course, worth discussing.

I ultimately decided it was problematic because that element, regardless of accuracy, was unnecessary in the story I was trying to tell, and its use -- by me the writer -- was therefore casual and hurtful. As others have noted there are some WWII stories that absolutely do require that sort of language, but this wasn't one of them.