Polish banners bought for the UK market

This is very interesting: a banner advertising O2’s UK mobile phone service, in Polish. There have been a few stories recently about media focusing on the Polish market, including a local paper in Reading publishing a Polish edition as a trial. The price is in pounds, so this isn’t a Polish banner that’s lost its way: it’s intended to be served in the UK.
What I’m interested in is how the media buyers decide where to buy this campaign, and what languages to use for which campaigns in which sites? The site I caught this on isn’t one devoted to a specific national group, the content is in English, so what led to the decision to serve a Polish banner ad?
The UK is so international today (which is a good thing) that I’m now wondering what languages I might see next. And how those of us who work in online advertising ought to be communicating with a multi-lingual international audience.