What language would you happen that has both ridiculous portability (you can use it from any language) and performance? Sounds like a fairy tale to me.
Go doesn't embed at all, so right there that's out.
Rust is still very unstable, and I wouldn't recommend it for production use.
I'm not sure why you would use Swift unless you're targeting iOS, it seems like a terrible idea to base your code on proprietary languages without ridiculous, .NET levels of support. I also don't believe swift is easily embedded with its huge runtime ass.
I'm not sure that Swift belongs on that list. It was announced back in June and there still no compiler for any non-Apple platforms. It's safe to say, at this point, that portability isn't one of Swift's goals.
Interesting question. I'm not intimately familiar with D, but it does seem like it's designed as a saner alternative to C++. How many C++ alternatives are out there? From my memory D seemed like one of the first major attempts to get away from C++. Maybe they didn't go far enough to meet peoples expectations. Or maybe they were too far ahead of their time.
Anyway, when you said that the first thing that popped into my mind was Ewan McGregor going all "you were the chosen one" and actually I think that kind of describes my feeling about D. At first it sounded very exciting, but then it seemed like there really wasn't a payoff and I ended up feeling kind of bleh about it. I wonder how many other people feel similarly. Maybe D's problem is kind of an emotional one?