When Antennapod requests the episode the server can inject the ads and geolocate you based on your IP. Thus they can tailor them at the point of delivery to your region (and anything else they know/guess about your IP). Antennapod does not inject ads itself, it is at the mercy of what the podcast server returns.

There has to be some kind of geolocation going on, when I download podcasts while traveling using antennapod commercials are often in the local language.

Podcast ads are dynamically injected at the time of downloading by the podcast host, based on factors such as the IP of the device downloading, and whatever other fingerprinting it can figure out at the time of download. You can see it in action by enabling a VPN through a country far away and notice the ads switching language.

AntennaPod is not adding its own ads (it's non-commercial, after all) and is not doing anything to help advertisers in their ad targeting.

The podcast provider themselves use download IP to dynamically grab ads and push them to your player.

It tags me wrong for some reason so I get French commercials even though I am in the Flemish part of Belgium.