The word “amen” is what’s known as a transliteration. A transliteration happens when a word is taken from another language, as is, to mean the same thing in the new language as it did in the language from which it was taken.
The New Testament was originally written back in the first century, in Greek. Greek was then the "second language" of the world, much as English is today.
But "Amen" in the Greek was actually a transliteration from yet another language, Hebrew. It was also transliterated into the language Jesus used every day, Aramaic.
“Amen” means truly! It's a word that underscores the truth of a statement that comes before or after it's said. Or, as in the case of prayer, it denotes faith in the God to Whom prayer is offered.
When in English translations of the Bible, we see Jesus saying things like, “Verily” or “Truly,” it means that the original Greek cites Him as saying, “Amen” or even, “Amen, Amen!”
Of course, as mentioned above, Jesus’ everyday language was Aramaic. But He and the fishermen and tax collectors among His disciples, given the cosmopolitan region in which they grew up, probably were conversant with Aramaic, Hebrew (the language used in the synagogue), Greek (the international language of trade and scholarship), and Latin (the language of their Roman conquerors). His earliest followers composed the books and letters that now make up the New Testament.