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]
Asked my boss to relocate me to another country 5 years ago. It was a once in a lifetime opportunity and I’m glad I even asked in the first place, because no one else had done it at the time.