What is the one form of communication that is understood by every culture and in every country?

A smile.

IN ADDITION:

Sign language. Most societies on earth use sign language to communicate. The American Sign Language is not known world wide, but with the lack of communication barriers of language, it's one of the most popular.

Answer

Music is considered to be the international language that unites all nations. Good music, played well, tells the same stories in all countries and all cultures. Music breaks down walls. On 29 Feb 2008 the New York Philharmonic Orchestra performed in Pyongyang, the capitol of North Korea. Among other music they performed the Korean national song and the US national anthem, The Star-Spangled Banner; to rapturous applause. That's communication.