Assimilation
"Germanic" as understood today is a linguistic term. Modern ethnicities speaking Germanic languages are not referred to as Germanic peoples, a term of historic scope. All present-day countries speaking a Germanic language including Germany have mixed ethnic roots not restricted to the Germanic peoples.
Germanic peoples were often quick to assimilate (although the term absorption could also accurately be used to describe several of the following historical situations) into foreign cultures. Established examples include the Gallo-Roman Norsemen in Normandy, and the societal elite in medieval Russia among whom many were the descendants of Slavified Norsemen (a theory, however, contested by some Slavic scholars in the former Soviet Union, who name it the Normanist theory).
The Germanic settlement in England is considered by many an example of assimilation, where elements of indigenous Celtic speaking Britons culture assimilated into that of the migrating Angles, Saxons and Jutes, resulting in an English identity for the inhabitants of that land. The later (mid-11th century) arriving French-speaking Norsemen similarly altered what was known as Anglo-Saxon England and set the English language on the path from Old English to Middle English.
It should be noted that the Normans had spent 155 years or approximately 7 generations in a predominantly Gallo-Romance France[citation needed] before proceeding to invade Britain. During this time they replaced their Nordic language with a middle French/Gallo-Romance language. Since the Normans are known in particular for their ability to merge with existing populations of people it can be assumed that they would take wives from local populations and become absorbed by the lands they settled within a few short generations.
As in England, Scotland's indigenous Brythonic Celtic culture in the southeast succumbed to Germanic influence due to invasion; while across the rest of Scotland Gaelic language and culture spread replacing Brythonic, primarily Pictish, languages. The Brythonic language survived for a slightly longer period in the South West of the country, principally under the Kingdom of Strathclyde, before succumbing to Gaelic pressure as the region was absorbed into the Kingdom of Scotland. Later almost the entire Scottish Lowlands became Scots speaking as the language displaced Gaelic over a period of some centuries. The Scots language is the resulting Germanic language now spoken in Scotland and similar to the regional variation of English in the north of England, Geordie (or Northumbrian) with which it shares a common origin. The Orkney Islands and Shetland Islands, though now a part of Scotland, were historically part of the Kingdom of Norway and Norse linguistically and culturally following the Viking invasions although plantations of English speaking Scots led to the death of the Scandinavian dialects in the 18th century.
France saw a great deal of Germanic settlement. Its namesake, the Franks, were a fusion of several Germanic tribes whose homelands lay along the Roman Rhine frontier, and who had been strongly influenced by Roman culture. Entire regions of France (such as Alsace, Burgundy and Normandy) were settled heavily by Franks, contributing to their unique regional cultures and dialects, and Frankish kings ruled the country from the 6th century to the 10th century. However, most of the languages spoken in France today are Romance languages, while the people have a heavy Gallic substratum that predates Latin and Germanic settlement.
Portugal and Spain also had some measure of Germanic settlement, due to the Visigoths, the Suevi (Quadi and Marcomanni) and the Buri, who settled permanently. The Vandals (Silingi and Hasdingi) were also present, before moving on to North Africa. Many words of Germanic origin entered into the Spanish and Portuguese languages at this time and many more entered through other avenues (often French) in the ensuing centuries (see: List of Spanish words of Germanic origin and List of Portuguese words of Germanic origin).
Italy, especially the area north of the city of Rome, has also had a history of heavy Germanic settlement. Germanic tribes such as the Visigoths, Vandals, and Ostrogoths had successfully invaded and sparsely settled Italy in the 5th century AD. Most notably, in the 6th century AD, the Germanic tribe known as the Lombards entered and settled primarily in the area known today as Lombardy. The Normans also conquered and ruled Sicily and parts of southern Italy for a time. Crimean Gothic communities appear to have survived intact until the late 1700’s, when many were deported by Catherine the Great. Their language vanished by the 1800’s.
Germany itself, during the last centuries BC, was mostly occupied by Celtic and Nordwestblock tribes who were linguistically assimilated into the Germanic peoples expanding from the western Baltic littoral area, as well as speakers of Romance languages in the south and west of the country and in Austria and Switzerland. In medieval times, under the identity of the Holy Roman Empire, Germany assimilated Slavic and Baltic peoples to the east (Ostsiedlung); after World War II their descendants spread to other parts of Germany as Vertriebene.
http://en.wikipedia.org/wiki/Germanic_peoples