Invented slang in a foreign language?

There have been a few threads elsewhere on the forums about how to deal with foreign language dialogue and translations in general, but I'm putting this one here because of the context:

I have a WIP set in the future, and because of that, I have some idiosyncratic language that I invented for the time frame and Other Worldness of it. However, I do have a handful of characters from what used to be Germany, and I'm wondering, when I write their dialogue, if I can keep that language, of if I should find an approximation in German for them to use? I'm writing most of their dialogue actually in English, with a few hints in their scenes that let you know that's not actually what they're speaking (based on the advice of one of the other translation threads) but this is a problem I hadn't exactly foreseen.

Help?

@Maraxus: No, you're getting it I think. (And thanks--I can't spell that word to save my life. I just know there's a Y in there somewhere. I blame dyslexia.) That's not a bad idea--they'd have a few different expressions anyway I reckon.

@maxmordon: I appreciate the input, but no. That's just lazy. It's not a whole new language, just a few words here and there.