As English is the international language of the world, why is French on this form?
I'd suggest starting with history. When many international organisations were formed, the international language was French, not English. It's really only in the 1990's that English started to really become 'international' after the Soviet Union fell and the role of Russian was eliminated.
Anyway, with the Universal Postal Union, the official language is French, not English - English is just a "working language". Therefore, the documentation should be in the native language and French. English is an irrelevance.
Also...on this form "CP72" why are there only 3 lines to write the articles in the package?
Again - it's a UPU issue, nothing to do with Poland. The forms are standardised.
teflcat - it could be Royal Mail - I've had quite a few things not even register as arriving in Poland after being sent through the Royal Mail - forums are full of people complaining about their international delivery too. Things seem to be going from bad to worse for them!