“Is the Orthodox one the only true one since it understands and prays in Greek?”
In Greece it is, it being the state church. Proselyting for any other religion can bring arrest and jail.
All in the name of Christ though, you see.
Have you ever read the Greek constitution? I doubt it. Nothing in it establishes Eastern Orthodoxy as the "state" religion. Church and state are separate. It recognizes is as the prevalent religion (93% of the population).
The Greeks are among the most religious people in Europe. The number of people who believe in God is equal or higher than in the United States of America. Only Malta and Poland come close.
Compare that to, say, Germany, where 30% are declared atheists, and church attendance is a meagerly 5-6%.
Orthodoxy is deeply intertwined with Greek culture, so that much of folklore also has Orthodoxy in it. The two are inseparable, as Judaism is inseparable from Israel.
As to being arrested for proselytizing, that may very well be true. Protestant Christians have no business in Christian Greece. Your missionaries should go where Christ is unknown.