Using an alternate Scripting Language to be interpreted by the browser instead of Javascript
One of the reasons you don't find browsers supporting languages like Ruby or Python, is because of the user's security. JavaScript was originally embedded in browsers for convenience, but, because that was an established relationship, as security issues occurred, JavaScript was locked down, as was the browser, to remove the capability of accessing the user's file system.
Other languages would have to be stripped of their ability to read/write from the disk before they would be allowed into a browser. 
Java applets can be created that can access the disk, but they're supposed to be signed to prove they're safe. A similar mechanism could be created for other languages, but since JavaScript and Java applets exist, I'm not sure the movement for supporting other languages would gain much support.
Can a scripting language like Python or Ruby replace Javascript as the browser's interpreted language so that we could be writing .py or .rb files instead of .js for frontend work? If so, would that be a good idea? If not, why? If it's a good idea, why isn't it done that way? If Python/Ruby can't replace JS in the browser, why not?