It doesn't matter what language a developer knows, they are all similar.
All procedural langs are pretty much the same. Functional langs are significantly different. Skill with applying OO concepts translates across languages pretty well, so long as you are cognizant of little differences which cause impedance mismatch. (Static method inheritance in Java vs. class methods in Smalltalk/Ruby.)
How many frameworks do I need to know?
One fewer than will fatigue you!