They say it was mainly Aramaic, sometimes Hebrew. But I want to think it Koine. Here's my article based on my mere assumptions.
I think Jesus was able to speak, use, understand workable Koine, not to say standard Greek.
I admit his native tongue was Aramaic, though, he must have understood Hebrew, at least what is written in Scriptures.
The reason he had to establish new religion somehow must have been related to his native tongue. He must have felt alienated from Hebrew dominated Pharisee society and Koine dominated Levant world.
That said, however, I assume his entire preachings were basically held in Koine.
What he wanted must be preaching as many people as possible.
His choice of universal language must have been the reason his teachings had dominated in Levant world, and his teaching itself matches the universality of its language.
