I was looking at all the answers already existing on this sub about the spread of Latin in the Western Roman Empire, the subsequent vanishing of pre Roman languages, and then the slow transformation of Latin into the various Romances Languages.
And well, at first, I found most answers pretty satisfactory.
But then I thought about my grandparents and realized that the point concerning the vanishing of pre Roman languages wasn't really clear.
In post Roman times, it seems that the only language existing in what are today Spain, Portugal, France and Italy is vulgar Latin. None of the pre existing Languages survived, except, for whatever reason, Basque. All other non-Latin Italic languages died, along with all the various Etrusques, Celtic, Hiberic, Gaulish, etc., languages. The Germanic languages existing in current countries that were, in Roman times, Latin speaking, arrived after the Fall of the Western Roman Empire. Same thing with Breton in Brittany.
I got that Latin was spread because it was the language of the administration, of commerce, and was mandatory if you wanted to move upward in Roman society, but is it really enough to result in the vanishing of pre existing languages ?
Let me explain : I am a Frenchman. And well, I guess you know it, until really recently in France, Parisian French was more of a sociolect that a language actually spoken by the population inhabiting France. So, while the nobility and really wealthy people living in important cities spoke Parisian French, regardless of where they lived in France, the vast majority of the population spoke only a really local language, and not Parisian French. I even saw figures saying something like 75% of the French population not speaking French well into the 19th century (don't know if it was really that high though).
Which was exactly the case for my grandparents. Unlike for me, their native language wasn't Parisian French, but respectively Breton, Basque and Alsatian (my grandparents all moved to Paris from their native regions), whereas they all were French citizen and their families had always lived in French territory.
And that's what made me realized that I hadn't read something really explaining how could Latin completely wipe out pre Roman languages in the Western Roman Empire.
I mean, if even well into the 20th century, in a world where technology, science, but also population density, means of transportation and more importantly means of communication are light years ahead of what existed in Roman times, allowing to spread information and languages way much more easily than in Roman times, there still were non Parisian French languages, how could Latin achieve to completely replace other languages in the Western Roman Empire ?
Not to forget that Parisian French, with the Ordinance of Villers-Cotterêts, was the official language in France for pretty much everything since the 16th century, but unlike Latin in the Western Roman Empire, didn't wipe out other languages. In fact you have to wait until after the middle of the 20th century to see all non Parisian French languages dying and Parisian French becoming the native language of pretty much everyone, and only because of the really aggressive linguistic policy of the French government.
But France isn't the only example making me wonder what happened with Latin in Western Europe. If the French and English ex colonial Empire allowed the spread of French and English in Africa, they didn't wipe out, at all, the native languages. There is still an incredible linguistic diversity in theses former colonial Empire. When Arabs conquered all of North Africa, the various Berber and Kabyles languages never died, they're still widely spoken in some area of Maghreb. The Ottoman Empire had Ottoman Turkish as the ruling language, but never wiped out all the other languages pre existing, and the fact that everyone in current Turkey only speak Turkish (well there's still the Kurds actually) is a recent phenomenon, only allowed by various genocide and population transfer. Even Franco's Spain and Mussolini's Italy, albeit violent dictatorship, had only really limited success in spreading a "standard" form of language across their country and destroying regional languages and identities.
When the Normans came to England and completely wiped out (I love that word) the Anglo-Saxon ruling class, and made French the language of the ruling class, they indeed greatly transformed the English language, but English itself never disappeared, in fact it even came back a few centuries later as the dominant language in England.
Well I think you got what I mean. Overall, it seems that the replacement of multiple language by one other on a great scale, both in terms of population and surface area, leading to a more or less homogeneous language continuum like it existed with Vulgar Latin in the Western Roman Empire and the death of any other pre existing language is a relatively new phenomenon, and that no country really achieved to do it before the 20th century.
So why didn't the Language spoken by Vercingetorix survive ? Or why didn't something similar to what happened in England post 1066 happened, i.e., a conservation of pre Roman languages but heavily modified by Latin ?
What made Latin in the Western Roman Empire so exceptional ?
I agree with the earlier, excellent, post that reminded us that native languages survived. I would like to point out however, that Ireland and Scotland were not conquered by the Romans (even if Romans like St. Patrick visited). For an example of the survival of native languages within the empire, as late as the 20th century, Breton and Welsh were spoken and mutually intelligible on opposite sides of the Channel. Native languages are also prevalent in toponyms. Take a look at the names for many cities in Iberia. The Basques also retained their own language, and were notoriously difficult to subjugate and assimilate (some things never change).
Regarding English, it's interesting to note that that language was also an "import" to the British Isles.
All of this, however, skirts your essential question: why did Latin "take over" a huge chunk of western Europe and retain its descendants? I think it comes down to two factors. First, what is now France and the counties of the Iberian Peninsula very early; prior to the fall of the republic. This relationship to power is going to affect language adoption and use. Think about how easy it is today for someone outside of the Anglophone world to learn English. The immense economic power of the British Empire and the geopolitical strength of the United States during the post-war period needs to be seen as the prime motivator of this phenomenon. I think we can see the Roman Empire in the same way. Much has been made of the relative disunity of the Roman Empire and its nature as a commonwealth, but I still think imposition of Imperial rule was of immense importance here.
In the case of eastern Iberia, Rome had maintained a presence in places since the end of the third century as a result of the Punic Wars. Think of the aforementioned modern analogue. How do you think the imposition of Latin language administration (and tax collectors, always important), the use of Latin in all official civic events (religious feasts, public games, local elections), and participation in long distance trade would shape language adoption? Regardless of the degree of provincial incorporation, I think this sort of linguistic power dynamic is the clear answer for the spread of Latin.
This brings me to my next point, and a response to something implicit in your question. Why did Latin persist when it no longer became the language of power? Latin had replaced local languages when the Roman Empire rolled in, why did Frankish, Gothic, Lombard, and others not replace Latin?
There are three main answers to this question as I see it. I can furnish citations for any of this if anyone is curious, but I would have to look up some old papers or something, and I'd rather not do that at the moment. Also, this response is going to be fairly strait laced, but I still think these phenomena are of paramount importance to the development of medieval Latin Europe, and interested readers can probably see where I come in on several historiographical debates.
First, the Germanic "tribes" that migrated into the Roman Empire would have been a fraction of the total vulgar Latin speaking populations in any given territory. Even of the Goths that smashed the Romans at Adrianople, for example, had been peacefully integrated into the Roman Empire wholesale, this population would have been surrounded by Latin speaking administrators, traders, religious officials, farmers, everyone. Again, I think the sheer relationship to power that many of these migrants would have experienced would have compelled bilingualism. The other thing to note about this period is that the powerful Germanics who replaced or became enmeshed with the local Roman aristocracies wanted to join the Roman enterprise and participate in the "Roman experiment" to facetiously use the terms of American history. Christian apologists Orosius's certain invention of a quotation attributed to the Gothic leader Athaulf is illustrative of this. Translation taken from wikipedia, but is cited:
"At first I wanted to erase the Roman name and convert all Roman territory into a Gothic empire: I longed for Romania to become Gothia, and Athaulf to be what Caesar Augustus had been. But long experience has taught me that the ungoverned wildness of the Goths will never submit to laws, and that without law a state is not a state. Therefore I have more prudently chosen the different glory of reviving the Roman name with Gothic vigour, and I hope to be acknowledged by posterity as the initiator of a Roman restoration, since it is impossible for me to alter the character of this Empire."
Finally, and to adopt the most traditional of explanations, the Church used Latin. We think of the use of Latin by the Vatican as archaic today, but it should be remembered that the Vulgate, the version of the Bible heard by every Christian obedient to the See of Rome since the time of St. Jerome is written in clear, simple Latin. Jerome meant for his translation to be accessible to everyone/speakers of vulgar Latin.
Additionally, Latin was not only used in Church ceremony, it continued to be the language of learning. The Germanic aristocrats who were inclined to learning would have certainly known Latin or a Latinate language, and the churchmen obviously knew Latin.
A few random points on this question. I don't think anyone knows the complete answer to this.
Some experts argue that Breton did not originate from Britain but is actually the last survivor of Gaulish.
We are talking about a 2000-year process of romanization here. Many of the pre-Roman languages that are now extinct may have survived to the Middle Ages, and we wouldn't know about it, since the written record of these times reflects only official and trade languages.
The Franks and Goths who succeeded the Romans decided to continue to use Latin as their official languages. If they had not made this decision, the outcome would probably be different.
While you bring many counterexamples, Latin is certainly not the only example of language change. Most of the American Indians in the western USA lost their languages within 100-150 years after they were conquered. Irish became almost extinct within 400 years after Ireland was conquered by England. Numerous languages went extinct in eastern Russian since these areas were conquered in the past few centuries. Chinese, Japanese, and Korean are all spoken in vast areas where none of the earlier languages survive.
While this may not fully answer your question, it is important to note the difference between dialects and languages. While, as given in your example, alsatian was not equivalent to Parisienne french, it was to a high degree mutually intelligible. This is called a dialect. It is a lot harder to erase a dialect than it is to replace a language as there exists a high degree of mutual understanding. Even in Roman times common Latin was far from unified and had several dialects, though it replaced the unintelligible (to Latin ears) germanic languages.