I am not sure that we are saying anything different from one another, at least not at the beginning.
I wasn't saying that the word came from Jerome's translation but from a Latin word. The term was used in Catholic catechesis and liturgy, and it became Anglicized as "Gentile." I was talking more specifically about the etymological history of the term. I wasn't referencing the Vulgate or current Neo-Vulgate and claiming either Jerome's or the current Holy See's text used "gentilis."
But it is also a falsehood that a word-for-word translation is more precise or correct than a dynamic approach. I speak several languages, including Bibilical Hebrew, Koine Greek, and Ecclesiatical Latin. No one translates or can translate one language into another in a true word-for-word manner, not even the NASB or NWT. All words have to be rendered contextually to at least some degree because of restrictions due to differences of idiom. There is no such thing as a true word-for-word version, and those that attempt to produce translations like this often give the worst renditions.
The LXX is not a word-for-word translation of the Hebrew, the Vulgate is not a word-for-word translation of the Greek, the KJV and the NASB are not word-for-word either. Only interlinear readings offer true word-for-word renditions, and if that type of translation was all we had to go by, we (or more accurately you) would understand even less than what current versions give you today.
The original Greek word in question means "nations," true, but are you reading the word "nations" like what the word means today or are you reading it as what it meant in the first century? The "nations" of the past were not secular like they are today. There was no "separation of church and state," so to speak then. Each nation had its national gods and was identified not just by a different culture but one heavily linked to the worship of national gods. So the word did not just mean a group of people under a particular government, but a nation of heathen or pagan people when compared or speaking to an audience of Jews. There were no secular states back then.
In Matthew the context is referring to prayers said by these people. Religion and religious practice is central to the identity of these "nations" in this verse because their repetition in their prayers is the subject matter. So Jesus is speaking not just about secular ethnic groups of people, but religious ethnic groups, heathens or pagans from the standpoint of the Jews. From the view I would say the NWT is not being accurate enough if it just uses "nations" since most today read that word without knowledge of the connection to religion that each ethnic group had in those days.