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Hello ! I'm from Nantes, so here are some quick answers to your questions.
Hanging out in the bars near Bouffay is pretty effective to meet new people. There's an Irish pub with a lot of english people and french people willing to speak english. It's pretty crowded, but there are tons of bars around so you'll find plenty of options. Google Maps link There's also the Buck Mulligan's pub not far from there. Basically, it's where you want to go if you want to have a good drink and have fun. Also, the Hangar à Bananes has some bars/restaurants too.
Knowing a little bit of french is definitely a nice thing, even if you're not fluent.
The local public transportation company is called TAN (Transport de l'Agglomération Nantaise). They have an app for both Android and iOS, but on Android I prefer NaonedBus (but unfortunately, it's down/unavailable until a few months, follow the news here and its developper here).
You can also use Uber to get a quick transportation, but it can be pricey (the app has an estimate feature). You can also rent Bikes (Bicloo) and Cars (iMarguerite) "on the go", but I'm not sure if you can if you're a foreigner since they may require a french credit card.
I'd say anything in a 2km radius from Place du Commerce is nice I'd say; the closest, the better (but the pricier). It should be close enough from everything and you could find housing for almost any budget. Nantes is relatively small, so the farther from the center you go, the less "active" the city is. Center is definitely the "Bouffay/Commerce/Quai de la Fosse" area.
If I had to "avoid" a part of Nantes, it would be the North-West. It's were more "popular" people live and some places can be not that reassuring at night, but it's not like some suburbs of Paris.
If you have any more questions, feel free to ask or send me a PM (so maybe we can talk on Skype or something !)