Why they speak French there and not here
I once gave a tour in downtown Zurich and mentioned that the Romans (who were after all here for about 400 years) left practically no traces of their presence. Other towns in Switzerland have magnificent Roman amphitheatres (Avenches, Martigny) but Zurich only boasts the vestiges of the Roman baths by the Weinplatz (Thermengasse). They didn’t even leave us their language, I casually said to my tour group. And that set me thinking: if the Romans were here for so long, why do we speak a Germanic language and people less than just 200km away speak a Romance language?
After consulting a few books from Zurich’s well-stocked Zentralbibliothek, I think I may have found an answer. It is a fact that the Roman Empire spent considerable time and energy protecting its borders. Most of the emperors were former generals who had risen to power following successful military campaigns at the limits of the Empire. The Romans were also very good at integrating foreigners into the Empire. One of the ways the Romans secured their borders was by co-opting the tribes that pushed against them. They gave them land and money, often their leaders’ children were kept in Rome as hostages and learnt Roman ways. Their leading warriors could be invited to fight for Rome against aggressors elsewhere (Germanic tribes would be sent East, Eastern warriors came West). And over time, the prospect of becoming a Roman citizen was desirable and made possible; speaking the language of Rome was a pre-requisite for integration. So, a lot of fierce, war-like tribes became a buffer for the Roman Empire and then tied their lot to that of their Roman masters.
By Marco Zanoli (sidonius 13:20, 18 June 2006 (UTC)) - Swiss Federal Statistical Office; census of 2000, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=875011
When the Romans pulled back suddenly from their northern limits at the beginning of the 5th century because armed tribes had penetrated into the heart of the empire and threatened Rome itself, the Alemanns, a Germanic tribe different from the Frankish tribes, were only just arriving into the region of southern Germany and what is now Switzerland. They occupied the space vacated by the Romans and established themselves here but had never had the time to become properly “civilized”. They obviously brought their own language. They were nominally subject to the ruling Franks farther West, who had adopted many of the Roman customs and even their language. The common people under the Franks eventually spoke a corrupted version of Latin that developed into French and other local variants. Incidentally, the extent of Alemannic influence on what is today southern Germany and Northern Switzerland is betrayed not only by the language but also by the names of towns ending in –ingen – Schwamendingen, Wipkingen in Zurich, Tuttlingen and Villingen-Schwenningen in Germany – a typically Alemannic word for a settlement.
The increasingly independent-minded Alemann ruling class was eventually wiped out by the Frankish Carolingians in a massacre in 746. But the local people continued to speak their own Alemannic language. As they do today. While in the German-speaking part of Switzerland, high German is the official language and is what you see written on menus and street signs, in newspapers and school books, on the street people still speak their own language. And though to outsiders, even German-speakers, it is mostly unintelligible, each region in Switzerland has its own distinct dialect with variations in vocabulary and expressions.