"The German airplane is flying over the Italian capital."
Translation:Niemiecki samolot leci nad włoską stolicą.
Short answer: Google Translate is wrong.
Slightly longer answer: GT is not a translator where every grammatical rule was coded by hand. Instead, it's an algorithm that analyses existing translation in order to "figure out" what to do. Thus, changing one word may sometimes in some inexplicable way significantly change the output. GT is useful to understand the gist of something written in another language, not to provide all the accurate details.