For me, it was learning English. Although I do make a few mistakes here and there, I’m mostly perfect on it.
The amount of resources you have access to dramatically increases when you know a universal language. I say dramatically because it made me realize how much my native language lacks when it comes to certain topics. The most obvious one to me was tech and computers. Everybody knows how to use Windows but there are very few resources about stuff beyond Windows. It’s actually sad. [insert sad face here]
Someone convinced me to study computer science 20 years ago. The career possibilities with good money and easy work have put the rest of life on easy mode (for now). Even meeting my wife and having my family would never have happend without that decision.