People think that there was an Anglo-Saxon invasion because the people of Britain suddenly changed their language and their clothes. This is not proof of an invasion. The people of Cornwall and Devon used to speak in Cornish then gradually started speaking in English as it spread down the length of Cornwall. This was not due to an invasion of 'English' people. The Cornish just changed their language.
English contains traces of the Celtic language in its structure as one might expect if a people started to adopt another language but did not speak it exactly how it originally was.
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