This isn't a game, there are no hard and fast rules. You can be successful in this world as a highschool dropout and you can live in abject poverty with a PhD in economics.
Learn JavaScript, or don't. In about a year, you'll be able to do Python and Ruby in the browser. I'm pretty sure there are JavaScript libraries that will do a conversion for you right now, but Mozilla is pushing to have native support for it anyway. I mean, what was the point of having the DOM if you weren't ever going to access it from anything but JavaScript? You can do C# and VB in the browser right now, too (SilverLight).
But really, learn every language. There is no "killer language" to learn. You should be learning them all. And once you get 10 or so under your belt, the others come with little effort. Languages aren't special. They're mostly all the same.
edit: and if you thought 6 years ago that PHP was the hot language and 3 years ago Ruby was the hot language, you have only been paying attention to your little corner of the world and need to get out more.