I studied Spanish before going to CR, and I loved being able to use it, but no - it's not necessary there. You'll hardly run into anyone - even in the middle of nowhere - that doesn't speak at least a little English. Tourism is the #1 commodity there, so it's to their benefit to be multi-lingual. Of course, it doesn't hurt to know some Spanish, and if you're going to be there for 7 weeks, you are definitely going to pick up a lot of it, especially if you try to immerse yourself.
As for the scene there - it's both. Ecotourism is HUUUGE, so a trip there can be all about the beaches and rainforests. But since that's generally only done during the day, the nightlife/party scene can be pretty hopping, depending where you are and what night of week it is. And it's not just the tourists partying. Ticos like to let loose and par-tay Thurs-Sat, sometimes Sunday nights. So any town you're in on those nights, there's bound to be a lively scene involving drinking and dancing. But you can really have it anyway you like. If the party/bar thing isn't your scene, don't worry, there are plenty of quiet places to go and reflect.
Check out the thread SV linked - it's got a bunch of mine and Voyd's experiences there.