The best way to incorporate other languages in a browser would be to have something similar to byte code running in a virtual machine (VM). As such any language that wants to access the browser/web API would need to be compiled first, unless there is an alternative way. Using a VM instead of embedding each language would help to keep the download size of each browser to a minimum. Naturally compile time would be the main bottleneck unless it is kept to a minimum. Now this would be a worthwhile challenge for Google (a fast performing browser VM)!
By incorporating a VM instead of embedding languages in a browser any language would be able to run in the browser (provided it runs in the VM), without requiring a separate runtime to be installed. For the future of web development this would be the next big evolution (not referring to Web 3.0) moving forward. If this is to work then the same system needs to be used by every major browser maker.
What browser makers will take up the challenge of incorporating other languages by implementing a cross platform virtual machine? Even though the challenge is great all it takes is for one browser maker to get the ball rolling.