The language is less important than your design choices. If you need to support tens of thousands of concurrent connections and want to do it efficiently you should almost certainly use a non-blocking event-based approach. Spawning thousands of processes (even threads) might seem convenient but it's also much less efficient and a headache in its own ways.

Take advantage of epoll under Linux and you can probably use any language you want. Brad Fitzpatrick wrote a Jabber server (DJabberd) in Perl that can handle 300k connections in 1GB of memory. It depends what you're doing, but it's highly likely you don't need to choose a painful language for the sake of efficiency.