I presume you want to "do the smart thing" by ensuring you will have good references.

However, were they laying you off, would they extend just because you threatened to say bad things about them? Hell no! They would fire you immediately and lawyer up. Look, at the end of the day you got to live with yourself. I would not stand for this. If he is going to try and strong arm you, then f*ck 'em. Leave immediately.

In the future, don't give notice until you've been offered the new position. It avoids all sorts of problems, including your boss firing you on the spot for looking, OR pulling this kind of shit.

AND save up sufficient "go to hell" money.

As someone who has studied Mandarin, Cantonese, Malay, Italian, Spanish and French, I have two answers for you. First, yes, do learn another language while you are still relatively young and it is not as difficult. As you get older, brain plasticity decreases and it gets a lot harder to learn a language. The experience you get being able to understand another culture through their language provides benefits far beyond more opportunity and salary. It's life experiece. Do it! The second answer has more to do with you figuring out what sort of lifestyle you want to have. There are a lot of good suggestions about what language to learn - Spanish, Mandarin or French are all good. However, the most important aspect of whatever language you decide to learn is you have to use it, or it will atrophy. So, choose a language based on where you would like to work or who you would like to work with, then go there and use it.