Do you have a learning syndrome?
I dont think so, I learn when it's necessary.

Do you agree with the idea of focusing on learning a technology/programming language? Why or why not?
I think you need to stick to a programming language cuz you dont want to just keep relearning how to do simple things, you want to advance your knowledge. Something to consider is you're usually making some product for a customer who doesn't give a shit what the programming language is, they just want the UX to be great and to accomplish their goal.

How do you make sure that you adequately learn programming languages?
When working, I shouldnt be hindered by my knowledge of the programming language, but rather just deciding the best way to architect the solution.

As for the full stack vs front/back end - I think this is partially an issue of the start up buzz these days. Everyone wants to make something, so you need to be able to do both, but if you work at a company you won't be able to create the product that quickly if you're doing both the front end and back end, specializing will be faster.