“How can they hear the gospel if the message is in Latin?”
The Gospel is read in Latin at the altar and THEN in English or the Lingua franca of the country from the ambo.
The “sermon” as you would call it, we call it the homily, is given in English OR in the Lingua franca of the country.
I guess that would satisfy the requirement. What may I ask, would be the purpose in that case, of saying it in Latin at all?