German-French Language Border
The development of the Franco-German language border can be traced back rather well. This history starts with Celtic tribes, who inhabited present-day Switzerland centuries before the arrival of the Romans. Little of this language has survived, apart from the tracing of names of persons, villages and fortresses (oppida). There was no written culture, but scholars assume a kind of Celtic cultural and linguistic unity. That changed with the establishment of the first Roman Colonia (Nyon and Augst, c. 44 B.C.) and the occupation of the territory of present-day Switzerland by the Romans at the end of the first century B.C. Within two generations the romanization of the elites of merchants, administrators and landowners was a fact, and the Roman writing culture became dominant in government, law, education, religion, trade and art. Various groups of the population also romanized, but often with the preservation or mixing of Celtic culture and language. That is also referred to as Gallo-Roman. There was no geographical language border in Switzerland, but there was a social gap between a fully Romanized elite and other inhabitants.
After the (slow) decline of the Roman Empire from the third century to the end of the fifth century (the formal end of the Roman Empire is 476 AD) the linguistic division of Switzerland did change. In the eastern part of Switzerland Germanic (Alamanni) tribes penetrated deeper and deeper into the country. Alamanni introduced the Germanic language, and by the 8th century, Gallo-Roman had largely disappeared. In the western part, a new kingdom was founded by the Germanic tribe of the Burgundians, who assimilated with the local Gallo-Roman population however and who even adopted the language. Then the area came under the control of the French-speaking Franks, with a language that is now referred to as Franco-Provençal. It was this period between the 6th and 8th centuries that the German-French language border was primarily established.
After the conquests of German-speaking Bern and Freiburg in the sixteenth century, the language border would shift somewhat, as in Wallis the language border shifted westwards in the fifteenth century by German-speaking bishops in Sitten (Zion) and German-speaking immigrants and military interventions. The supremely powerful Bern (and allies) not only defeated the French-speaking Duke of Burgundy in 1476, thus gaining enormous prestige, but also conquered the French-speaking Waadt from the French-speaking Duke of Savoy. In 1536. Areas sometimes became bilingual, such as Morat (Murten), but usually not much changed, because the German-speaking rulers respected the French language and even communicated in it themselves. The elite of Bern even elevated French out of prestige to her language of address, Leurs Excellences Messieurs.
Freiburg also became ruler of French-speaking areas (Gruyère, Bulle, Romont, among others), but this led to a Frenchification and bilingualism of the originally German-speaking Freiburg and Freiburg became more and more Fribourg. The basis of the French-German language border was laid in the 6th-8th century, that language border shifted somewhat to the west, and some cities became bilingual, but the centuries after the fall of the Roman Empire were decisive (A. Paravicini and others (Red.), Les Pays Romands au Moyen Age, Lausanne 1997).