The RSS-language will change the tag of your feeds from the default (‘en’) to the language of your WP installation by default. In case you are e.g. using French as the WP language, but want to publish in Dutch, you can change the language in the settings section. Nothing specific will change in the feed but your blog will be found easier by people using the language, you are actually writing in. Also it helps search engines to list your site correcly, if you provide the feed as a sitemap.
The plugin was tested up to WP 3.4 and should work with versions down to 2.7 but was never tested with those.
