I travel a lot and have wondered about that. I'm often seeing ads in the local language (based on my IP), not the language of the site or the language set as default in my browser or OS. Makes absolutely no sense, they'll never be clicked since they are unreadable... the ultimate in untargeted ads, just a waste of an impression and bandwidth.
Warning, rant ahead:
A similar pet peeve, why do so many sites automatically redirect to the local language of the visitors IP? (Bing, I'm looking at you! If I search in one language, give me results in that language!)
Have you ever tried to navigate to set the language when you can't read the displayed language to find the link? It's even more fun in countries that don't share the same alphabet. :/
Yes I know, use a VPN. I usually do but sometimes it slows the connection too much... try having dial up in a developing country slowed further. Sites timing out doesn't help the language barrier either ;) And I'm not going to do that just to have readable ads.
Pardon the rant, needed to get that off my chest.