Normally, a court cannot listen to testimony from a translator, as this has the same status as hearsay (eid mipi eid). So, what is a sanhedrin (specifically a minor sanhedrin, but I don’t see any reason why this shouldn’t apply to the Sanhedrin as well) in a capital case to do when they receive testimony in a language they don’t understand?
The answer (based on the Margaliyas Yam and K’hilos Ya’akov) is that three of them who do understand the language form a bais din on their own and listen to the testimony. The beis din can then issues a p’sak din about the testimony they received. The sanhedrin can then use that p’sak without it being a case of eid mipi eid.
That being said, there was no need for every member of the Sanhedrin to speak all languages. Rather, you simply needed to have at least three speakers of all the languages among the group.
The Wolf