I agree that  behaviorism has a lot of problems, and can't account for all of language.  However, rewards and punishments can be a big short-term motivator for long-term learning.  I have seen young children who would gladly commit bloody murder if they thought it would earn them a sticker from their teacher.  People will put insane amounts of work to change their avatar in a learning app or watch a lazily-animated three-second fireworks display. 

But of course rewards and punishments aren't the only components of Skinner conditioning: there's also the formation of  an immediate, unthinking association between the reward or punishment and the target action.  This is useful too.  When I click on an answer I'm not sure about on a language-learning  site, my shoulders tense and my adrenaline spikes while I wait to be punished with a flashing red "X".  Is that enough to make me learn a language?  No.  But it provides a nice bit of short-term motivation to supplement my long-term goals.