For the linguists, an interesting tidbit I stumbled upon.
What is know of Gaulish and Lepontic Celtic shows that it was very similar to Latin. The syntax and grammar were apparently almost identical and many words very also identical or similar enough to be intelligible.

For example, the Gaulish word for "horse" is equos and the Latin is equus. Divine is divo in Gaulish and divinus in Latin. King is rix in Gaulish and rex in Latin. Gauls drank cervesia ("beer") while Romans had cervisia. Mediolanum, Milan's ancient name, can mean "middle plain" in either Celtic or Latin.

Some words seem to have their letters jumbled from one language to the other. The Latin mundus ("world") is dumno in Gaulish. A bull is taurus (or tavrvs as Romans didn't distinguish "v" from "u") in Latin, but tarvos in Gaulish.

Many Gaulish words have closer equivalents in Germanic languages than in Latin or modern Romance languages. "iron" was isarno in Gaulish, which relates to the modern German Eisen, Gothic eisarn and Old English isern or iren, but not at all to the Latin ferrum. In all likelihood it is Germanic languages that imported the Celtic term, as the Celts of the Hallstatt culture were the first to mine, work and export iron ore. The Gaulish word for mountain could have been perkun, related to the Gothic fairguni and the modern German/Dutch/Scandinavian berg(en). But this could stem from an older Indo-European root.

Naturally, Celtic, Italic and Germanic languages all stem from a common ancestor, the "Centum" dialect of Indo-European, most probably spoken by the R1b people who arrived in Europe from Anatolia along the Danube around 4300 ybp. The question is, does it make sense to divide them in three branches, or are Brythonic, Goidelic, Celtiberian, Gaulish, Lepontic, Latin and Oscan all part of a bigger Italo-Celtic family ? There was probably more difference between ancient Irish and Gaulish than between Gaulish, Lepontic and Latin. Even nowadays Welsh, Cornish and Breton cluster together, but are a world apart from Irish or Scottish Gaelic.

Nicholas Ostler explains in his remarkable book Empires of the Word, A Language History of the World (, that languages are easily replaced by a closely related language, but rarely by a very different language unless their is a massive migration accompanying it. This is how Akkadian was quickly replaced by its cousin Aramaic in the Fertile Crescent, without invasion needed (Aramaic was the language of dessert nomads, as opposed to Akkadian that was spoken in the big cities). Yet Aramaic survived 1000 years of Greek and Roman rule, only to be replaced in a few decades by Arabic, another closely related Semitic language. The same happened with Egyptian, which the Greeks and Romans never managed to replace, but which was taken over by Arabic during the Muslim conquest. What was different ? Aramaic and Egyptian speakers could learn Arabic easily : same syntax, similar grammar, vocabulary and pronunciation. Only a mild adjustment was needed. It was more like learning a new dialect than a completely foreign tongue like Latin, Greek or Persian.

The same must have been true between Celtic and Latin. Gaulish, Celtiberian and Lepontic speakers could learn Latin fairly easily, without changing the word order in their head and starting with a lot of common words. It would be like for a modern Spaniard to learn Italian or French, which is much easier than to learn Greek, Russian or Japanese. It could even be argued that Vulgar Latin, which evolved in the modern Romance languages, was an admixture of Latin with the various regional Celtic dialects. This would explain some pronunciation changes such as the Classical Latin -us ending into the Vulgar Latin -o ending, just like in Gaulish. If this really did happen, French, Occitan, Italian, Catalan, Castillan, Galician and Portuguese would not be direct descendants of Latin, but blends of Latin with regional Celtic dialects, explaining most of the present regional variations. The dialects of Arabic also appear to have incorporated quite a lot of words from the languages that preceded it. Egyptian Arabic blended Classical Arabic with Egyptian, while Maghrebian Arabic has Berber as well as Carthaginian influences. As a result, Maghrebian Arabic is no more intelligible to someone from Saudi Arabia as Portuguese is to an Italian.

It is telling that the Romans never managed to impose their language on anybody else than the Celts, and not even the Britons among them (apparently their Brythonic dialect was too remote from Latin, compared to Gaulish or Lepontic). The Greeks, Syrian, Jews or Egyptians never learned Latin (except for a small elite). In fact, the Jews of Egypt preferred to communicate in Greek rather than in Latin. From the the division of the Roman Empire in 286 CE, Greek regained its former status of official language of the Eastern Mediterranean. The only other region where Latin was spoken by the majority of the population was Dacia (modern Romania), which was also Celtic before the Roman conquest, and was massively resettled by Romans from Italy.

The Basques are another good example of the difficulty of imposing a completely different language on a population. Although all the surrounding Celtic speakers quickly adopted Latin, the Basques, whose language is not part of the Indo-European family, retained their language up to the present day.

Germanic invaders of the Western Roman Empire adopted Latin because it was seen as a prestige language. They needed to speak it in order to administrate the conquered people who vastly outnumbered them. Speaking Latin also gave legitimacy to any wanna-be heirs of the Romans, be them the Goths, the Burgunds or the Franks. Germanic languages only survived in regions close to the Germanic homeland, where massive migration took place, like in England, Flanders, Rhineland, Bavaria or Austria.

In other words, Germanic languages must have been different enough from Latin to prevent the masses to learn Latin when they could live without it, and vice versa. Only when outnumbered did speaker of one language adopt the other. The few Roman Britons who didn't escape to Wales, Cornwall or Scotland had no choice but to learn Anglo-Saxon. The Gallo-Romans who stayed in Flanders or Rhineland had to learn Frankish due to the vast number of Germanic immigrants. But wherever the Gallo-Roman population exceeded the newcomers, Latin survived, sometimes after a bilingual period where both Frankish and Latin were spoken, like in Wallonia, French Flanders or Lorraine. Luxembourg has remained bilingual to this day.
The ensuing conversation clears up discrepancies and presents additional info.
Another article (PDF format) of interest linked in the thread