Skinner supposed that learning a new language is done through stimulation and reinforcement, do you agree with his theory? why

Any Language for that matter is a matter of Repeatation & Conscious  Practice .  So Re inforcement of Concepts  ,  yes it does helps however what  matters  is Consistency . 

It is a skill. One needs to get motivated. Initial stages of learning a language is bit difficult. That is the reason, it is taught with fun or through rhymes

Considering that Skinner was more than a little evasive os using the word  "language" I think that this irendering of his supposition is inaccurate. Having said that however, I believe that to a great extent linguistic ahievement or even shortcomings are heavily dependent on stinulation and reinforcement. I've noted various anecdotal cases in which quite proficient language users learnt a great deal via osmosis of passive television watching. I contend that they were not met with much reinforcement yet achieved functional status with some idea of register and appropriacy. 

I have no idea who Skinner is, the only Skinner I know of is the one from the Simpsons. Anyway, I think a language must be acquired via practice and more practice