Just my two cents - in a post-apocalyptic world where danger is everywhere, resources are scarce and the means to travel are severely limited, it's very unlikely that a significant amount of people would attempt, nevermind succeed, to travel to places distant enough that they wouldn't know the language. Not to mention, they would have little incentive to, seeing as being a forenigner unable to comunicate would make their situation even more precarious....language barrier
You could perhaps have groups of slave traders organizing raids in distant lands and bringing back foreign slaves, but again, why use so many resources (at the very least cars and fuel) when slaves are obviously a plentiful resource locally.