[dangerbird2]

English is member of a language continuum, albeit a disjointed one by virtue of the English Channel and the Norman conquest. The Ingvaeonic languages is a "gradient" including Scots, English, Frisian, and Low German/Plattdüütsch. Frisian is certainly legible to an Anglophone with a wide exposure to English dialects and historical periods.

https://en.wikipedia.org/wiki/Ingvaeonic_languages

[jacobush]

Don't forget https://en.wikipedia.org/wiki/Old_Norse and its influence on English. (I say) some of the contemporary Norwegian is very close to some contemporary Scots.

[TheOtherHobbes]

The English continuum also includes pidgins of various kinds, and Engrish.