Goodness me, that idea is solid gold. Imagine there not just being French, but some German, Spanish, Italian and perhaps even some Dutch in there as well. Imagine walking through the bar area and hearing all these languages!
As for subtitles, perhaps they could arrange it so that only the conversation 47 is looking it appears translated into English in the subtitles. That way you won't miss any information and you have all the immersion a Hitman map needs. It'd be very realistic too: even if you're a polyglot, you need to focus to make sure you understand everything that's being said during a conversation in a foreign language. It would add another layer to the assassin fantasy: discerning information in foreign environments.
In the Marrakech map, you'd mostly encounter Arabic and French. And Sandberg's language (was he Danish?). Perhaps a couple of Brits that are visiting from Gibraltar?
In the Japanese map you'd hear lots of Japanese, perhaps some English and Chinese?
The Indonesian map could pose a nice language barrier as I doubt 47's Bahasa is good enough to make out what's being said.
... but we're probably going to hear Brian Kimmet everywhere =/