Anyone who grew up in Bethsaida would not only understand Gk, but would also have been polished by dealings with foreigners and would have been accustomed to Gk. culture.FWIW (Volume VI, page 100)
In the same article Bethsaida is often described as an area under Greek influence.
I have no doubt that most everyone there spoke Greek, but the question becomes WHY would the speak Greek among each other when their native language was Aramaic?
And has you pointed out, the Gospel of Saint John clearly uses the Aramaic as do other portions of the New Testament.