Any server-side language will do since your getting this information from your server. PHP is the most accessible for the average person. You cant really use Javascript to implement the generation of an RSS feed. It will be a simple case of retrieving the content from somewhere, could be a databse or text file or whatever. Then you take this content and make it safe (meaning it doesn't contain any HTML characters that will distort the output) then output the feed in the format you choose to use placing the content in the place its supposed to be.
You can google for examples of this.