Spanish or Mandarin Chinese. You're not going to be using Korean or Japanese much as a doctor unless you practice in the actual country.
Out of those two, Spanish, by far, is much easier for someone who knows English already.
Japanese is very common in Hawaii from what I hear, and I've lived in areas where there were so many Koreans that many of the stores and restaurants only spoke Korean (and also only wrote their signs in Hangul). Likewise, Chinese is extremely common if you live in, say, San Francisco. Spanish is common everywhere though.
The thing about languages though is that it really helps if you find something inherently rewarding about knowing the language. While Spanish is the most useful second language to know for an American, I hated studying it throughout K-12 because I found it and the cultures of the countries it's spoken in to be extremely boring. As a result, I took Japanese for my foreign language GER in college and loved it, even though it didn't do any favors to my GPA and was responsible for 80% of my workload that year despite me taking nothing but science classes for the rest of my credits.
The reality is that you don't need to know a second language in this country to practice medicine. There are more than enough areas where English is the only spoken language. This is especially true if you plan on catering to the same demographics most doctors do. As a result, you're not going to feel a powerful motivation to learn a language just because it
might be useful
one day. You need something more; do you plan on often visiting the actual country or countries where the language is spoken? Do you really love the culture? Is there something about the language itself you find fascinating?
Just study whatever you want.
Also, as an aside, don't learn Chinese because "OMG they're gonna be the next superpower!" Just because they've been experiencing large growth does not mean that growth is going to continue forever. In fact, it's already slowed down considerably, and now some analysts are even predicting a crash (although I would take anything an analyst says with a grain of salt, most of them are idiots). This whole hype/paranoia about China going on to supplant the US as a superpower has been repeated several times before in the past, and the predictions never became true. For example, before China it was Japan people were convinced was going to overtake the US as the world's dominant economic and cultural power. Then the housing bubble popped and Japan's population growth issues exacerbated things, and ever since then Japan's economy has been stagnant.
