Create a custom feed based on custom field value

I’d like to offer language-based feeds for a multi-language blog. I could use a tag for storing language information but it isn’t the best solution since I use tags to describe post content and language is more like meta information. Is there a way to filter a feed by values of custom field with query parameters (like is possible for tags by using ?tag=tagname) or do I need to write a custom feed myself?