No, rather the endings. And -o ending is problematic: in theory it should undergo declension, but in so many examples it just sounds weird to Polish people...
For example, let's take Cristiano Ronaldo - until recently I had no idea that this name should undergo declension, I have never heard it or read it in any other form, but apparently it should be. For example "Cristiano Ronaldo's goal" should be "gol Cristiana Ronalda". But this looks as if his name was Cristian Ronald. I've only seen it once and it was an article on grammar.
(on the other thing, if we considered the real, Portuguese pronunciation, then both words end with -u and it is correct to not change them :D But how many football fans in Poland pronounce it correctly? Almost no one, I guess.)
Grammar. Also, having funny/weird sentences makes them easier to remember instead of just translating plain, boring sentences. And that's the point, that people should remember the grammar and be able to say what they want and not just repeat ready phrases. Those you can find in books known as "rozmówki" :)
Remember that the most famous phrase since this website was founded is "Yo soy un pingüino." - Duolingo even sells t-shirts with it ;)