Many years ago, the British Empire ruled almost half the world. Although they encouraged their extended citizens to learn English (which is how the language came to be so widely spoken), they did at least appreciate that not everybody would be able to do so. Also, as their role these days is largely to be global diplomats for Great Britain, it's beneficial for them to be able to speak as many languages as possible.
Teaching foreign languages to the Royal children starts at an early age. Prince George is already learning to count in Spanish, and by the time he's a young adult he should be fluent in Spanish, French and German at the very least. They should have an advantage with the latter; there are still traces of German ancestry within the Royal Family, and the Queen is known to be proficient in the tongue. If William and Kate also wanted George and their other children to speak Greek, then he could send them to spend some time with their great-grandfather Prince Phillip, who's a native speaker.