In short, both.
There are many other deciding factors taken into consideration. Ultimately, the specific ad is determined by the highest paying advertiser that actually buys the ad space. Location of your visitor and the site language are very important factors in this dynamic, but not the only ones.
Sulvo ads will generally show ads in the language based on:
User's browser language
Language of the content
User's location
Bids will likely come in from advertisers from all 3 targeting options, and the highest paying one will show.
To learn more about supported languages, please see the "What languages does Sulvo support for ad serving?" article.