Gaulish in the context of "Parisii (Gaul)"

Language shift, also known as language transfer, language replacement or language assimilation, is the process whereby a speech community shifts to a different language, usually over an extended period of time. Often, languages that are perceived to be of higher-status stabilize or spread at the expense of other languages that are perceived—even by their own speakers—to have lower status. An example is the shift from Gaulish to Latin during the time of the Roman Empire.

Language assimilation may operate alongside other aspects of cultural assimilation when different cultures meet and merge.

The Gauls (Latin: Galli; Ancient Greek: Γαλάται, Galátai) were a group of Celtic peoples of mainland Europe in the Iron Age and the Roman period (roughly 5th century BC to 5th century AD). Their homeland was known as Gaul (Gallia). They spoke Gaulish, a continental Celtic language.

The Gauls emerged around the 5th century BC as bearers of La Tène culture north and west of the Alps. By the 4th century BC, they were spread over much of what is now France, Belgium, Switzerland, Southern Germany, Austria, and the Czech Republic, by virtue of controlling the trade routes along the river systems of the Rhône, Seine, Rhine, and Danube. They reached the peak of their power in the 3rd century BC. During the 4th and 3rd centuries BC, the Gauls expanded into Northern Italy (Cisalpine Gaul), leading to the Roman–Gallic wars, and into the Balkans, leading to war with the Greeks. These latter Gauls eventually settled in Anatolia (contemporary Turkey), becoming known as Galatians.

Proto-Celtic, or Common Celtic, is the reconstructed ancestral proto-language of all known Celtic languages, and a descendant of Proto-Indo-European. It is not attested in writing but has been partly reconstructed through the comparative method. Proto-Celtic is generally thought to have been spoken between 1300 and 800 BC, after which it began to split into different languages. Proto-Celtic is often associated with the Urnfield culture and particularly with the Hallstatt culture. Celtic languages share common features with Italic languages that are not found in other branches of Indo-European, suggesting the possibility of an earlier Italo-Celtic linguistic unity.

Proto-Celtic is currently being reconstructed through the comparative method by relying on later Celtic languages. Though Continental Celtic presents much substantiation for Proto-Celtic phonology, and some for its morphology, recorded material is too scanty to allow a secure reconstruction of syntax, though some complete sentences are recorded in the Continental Gaulish and Celtiberian. So, the main sources for reconstruction come from Insular Celtic languages with the oldest literature found in Old Irish and Middle Welsh, dating back to authors flourishing in the 6th century AD.

The Helvetii (Ancient Greek: Ἐλουήτιοι, Latin: Helvētiī [hɛɫˈweːti.iː], Gaulish: *Heluētī), anglicized as Helvetians, were a Celtic tribe or tribal confederation occupying most of the Swiss plateau at the time of their contact with the Roman Republic in the 1st century BC. According to Julius Caesar, the Helvetians were divided into four subgroups or pagi. Of these, Caesar names only the Verbigeni and the Tigurini, while Posidonius mentions the Tigurini and the Tougeni (Τωυγενοί). They feature prominently in the Commentaries on the Gallic War, with their failed migration attempt to southwestern Gaul (58 BC) serving as a catalyst for Caesar's conquest of Gaul.

The Helvetians were subjugated after 52 BC, and under Augustus, Celtic oppida, such as Vindonissa or Basilea, were re-purposed as garrisons. In AD 68, a Helvetian uprising was crushed by Aulus Caecina Alienus.The Swiss plateau was at first incorporated into the Roman province of Gallia Belgica (22 BC), later into Germania Superior (AD 83).The Helvetians, like the rest of Gaul, were largely Romanized by the 2nd century. In the later 3rd century, Roman control over the region waned, and the Swiss plateau was exposed to the invading Alemanni. The Alemanni and Burgundians established permanent settlements in the Swiss plateau in the 5th and 6th centuries, resulting in the early medieval territories of Alemannia (Swabia) and Upper Burgundy. The Helvetii were largely assimilated by their new rulers, contributing to the ethnogenesis of modern Swiss people.

The Insubres or Insubri were an ancient Celtic population settled in Insubria, in what is now the Italian region of Lombardy. They were the founders of Mediolanum (Milan). Though completely Gaulish at the time of Roman conquest, they were the result of the fusion of pre-existing Ligurian and Celtic population (Golasecca culture) with Gaulish tribes.

Throughout Roman history, there was regional variation in the Latin language. In certain regions, terms or morphological features from pre-Roman substrate languages were borrowed into the local dialects. For instance, the dialect of Gaul borrowed terms from their native Gaulish language, which was spoken by the Gauls, a Celtic people. Regional dialects were often perceived as inferior to the prestige "Roman" dialect, which—in the view of some authors—may have constituted a genuine style of speech common to the city of Rome. However, other authors perceived their ideal "Roman dialect" as an artificial, prescribed standard of "correct" speech that did not necessarily reflect the vernacular of any given region.

Galatian is an extinct Celtic language once spoken by the Galatians in Galatia, in central Anatolia (Asian part of modern Turkey), from the 3rd century BC up to at least the 4th century AD. Some sources suggest that it was still spoken in the 6th century. Galatian was contemporary with, and closely related to, Gaulish.