Your best bet is to go with popular hostels. You might pay a bit more, but you'll avoid the traps that you can stumble into if you go for cheap and less heard of. Good reputations usually mean safer hostels, and so places like the Hotel Eldorado stand in a league of their own. The Hotel Atelier Montparnasse also has a good reputation, but not as established as the Hotel Eldorado.
Unfortunately, the French have a reputation for hostility towards those who don't speak their language. Don't despair, though, so much of that is a stereotype. Still, you can check out the Bombardier - an English pub, where speaking English will almost certainly not make you stand out. You may get a slight light-hearted jossing for being a "Yank," though. It comes with the territory.
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