Node JS routing system
It seems a little risky to me to just blindly use the language designation that the URL contains. If it's possible to trick Express into looking somewhere in your file system besides where you expect (perhaps with dots in the path), then you could have a security issue.  It would be safer if you checked to see if it was one of a list of known-safe designations ("en", "fr", "ge", "sp", etc...). If you wanted, you could build a list of known safe designations by looking in the the file system at server startup and then just check it against that.

I would also recommend using path.join() instead of adding in the path separator yourself.  That will make your code more platform friendly should you ever want to run it on Windows.