That does make some sense; Amazon only officially supports ebooks in 6 languages: English, German, French, Italian, Spanish and Portuguese.
But the really odd part of the tale is that not all of Piotr's Polish language ebooks were taken down on Friday. It seems that someone inside the Kindle Store noticed that this one particular title was written on Polish, even though the language was set to English. That faceless drone changed the language listed in the ebook's metadata and then removed the ebook.
The rest of Piotr's Kindle ebooks are also marked as being written in English, and Amazon missed those titles (even though one has the description and content in Polish).
Update: A friend commented on Twitter that Amazon has long had a policy of removing content sold in unsupported languages. This is true. I got interested in this particular situation because of the rumored launch of the Polish Kindle Store. Also, I find the haphazard enforcement rather strange.
Now, as odd as this may sound, I can probably guess why this happened. It's a technical issue, and it has to do with fonts and character encoding. There is a valid concern that an ebook sold in an unsupported language might not display all the characters properly. The Kindle does not have complete support for all unicode characters, so removing an ebook that might be using an unsupported character makes sense (in a bureaucratic sort of way).
Clearly this isn't actually a problem with Polish; Piotr 's customers could read his ebooks just fine. But I do understand why some unnamed drone in the Kindle Store pulled the ebook.
On a related note, this bit of news has thrown a spanner in the rumored launch of the Polish Kindle Store. Amazon appears to be discarding content just weeks before they are predicted to support it officially. That seems a little self-destructive, in my opinion.
In any case, this move is also shortsighted. There are a reported 60 thousand Kindles in use in Poland, even though the language isn't officially supported. That might be a small number in terms of the many millions of Kindles in use, but it is also a group that Amazon is actively discouraging from owning Kindle.