I just stumbled across an essay by linguist John McWhorter from 2015.
There is no other language, for example, that is close enough to English that we can get about half of what people are saying without training and the rest with only modest effort. German and Dutch are like that, as are Spanish and Portuguese, or Thai and Lao. The closest an Anglophone can get is with the obscure Northern European language called Frisian: if you know that tsiis is cheese and Frysk is Frisian, then it isn’t hard to figure out what this means: Brea, bûter, en griene tsiis is goed Ingelsk en goed Frysk. But that sentence is a cooked one, and overall, we tend to find that Frisian seems more like German, which it is.
English started out as, essentially, a kind of German. Old English is so unlike the modern version that it feels like a stretch to think of them as the same language at all. Hwæt, we gardena in geardagum þeodcyninga þrym gefrunon – does that really mean ‘So, we Spear-Danes have heard of the tribe-kings’ glory in days of yore’? Icelanders can still read similar stories written in the Old Norse ancestor of their language 1,000 years ago, and yet, to the untrained eye, Beowulf might as well be in Turkish.
The first thing that got us from there to here was the fact that, when the Angles, Saxons and Jutes (and also Frisians) brought their language to England, the island was already inhabited by people who spoke very different tongues. Their languages were Celtic ones, today represented by Welsh, Irish and Breton across the Channel in France. The Celts were subjugated but survived, and since there were only about 250,000 Germanic invaders – roughly the population of a modest burg such as Jersey City – very quickly most of the people speaking Old English were Celts.