Friendinflatbush
I’m assuming that anyone on the Sanhedrin was a genius and brilliant people with great memories can learn things rather quickly even language. Theres a story where the Rogatchover gaon who lived a 100 or so years ago was so brilliant that he said about himself that he could learn Russian in an hour! He was a huge genius. So it’s not so hard to imagine that the Sanhedrin memorized that many languages. As for the method I have no clue. I doubt that it is brought down anywhere either but I assume they had a method that would cut the bittul torah to a minimum.