Dialect in the linguistic sense of a variation of a language.
English, the language of the Angles foreigners who came to Britain, has left its mark on this Island. Ænglisc or English a Germanic language started out pure to a degree and went to Old English(written in Roman script). There are few people today, who can read Old English. From Old English it went on to Middle English and today we have Modern English. (I read and understand Old Dutch, as well as Modern Dutch.)
Today English has been more than 50% diluted than what it was. It is 26% of what it used to be.
The question of how divergent dialects must be to be considered different languages has never been resolved. As a guide, we might look to cognate percentages between what are generally recognized as distinct European languages. For example, Czech -Russian 74% English- German 60% English -Czech 25% (Fairbanks 1955:118)
Swadesh (1954:326) suggested that 81% cognates or better indicate that the dialects belong to the same language. Today: According to surveys, the percentage of modern English words derived from each language group are as follows: Latin ≈29% French ≈29% Germanic ≈26% Greek ≈6% Others ≈10% (https://en.wikipedia.org/wiki/Lists_of_English_words_by_country_or_language_of_origin#/media/File:Origins_of_English_PieChart.svg )
Bede writes: In the year of our Lord 449 A.D. the nation of the Angles, or Saxons, being invited by the aforesaid king( King Vortigern), arrived in Britain with 3 long ships, and a place assigned them to reside in by the same king, in the eastern part of the island, that they might thus appear to be fighting for their country, whilst their real intentions were to enslave it.