I have only been using WordPress for about a week so do not take my advice a definitive. Maybe you need to have one feed per language and to accomplish that you will need to put the articles for each different language in the same category. Then, advertise the feed for that language.
As I was looking around for a newsletter solution, I came across this, which describes how to make category feeds.
Apart from that, you would probably need to do some RSS feed autodiscovery but... how can the RSS readers know what language the user wants?
If you fix it, please post your solution up because I am also interested in having a multi-lingual blog.