The site language is a setting that tells Discourse to use different texts for each locale. These texts are included in your Discourse installation. Anything that a user/admin has manually put in does not change based on a language setting AFAIK.
A dumb thing I made ages ago was a PHP script that used a GeoIP service to find a user’s country, then automatically translate parts of the site text into the country’s native language using the Google Translate API.