I don't think Jack wouldn't use racist language because he's not an asshole; I think he wouldn't use it because it wouldn't occur to him unless he's gotten really used to the language from hearing other people use it all the time. Or maybe because of how Time Agent training works -- I don't know.

But I'd agree that the language isn't necessary for that particular story. Possibly it would be in a longer story about how surviving WWI and WWII messes with your head and changes what you think about what appropriate behavior is. (I love how in Foyle's War you keep running into people who are totally fucked up and you find out that they were at Ypres. Yes, that is correct. And now I'm thinking about Jack at Ypres, because not only could he have been one of the few who survived the battles there, he could have died during them too.)