I used to think that maybe English was a Celtic language. Or at least wonder. After all, it's right there on a pair of islands where most of the surviving Celtic languages come from. But no, linguists insisted it was a Germanic language with a lot of French and Latin borrowings. And it's true that English doesn't have a lot of confirmed Celtic cognates. One of the few common English words traced to Welsh is "penguin", which originally referred to far north seabirds like puffins and auks.
But Our Magnificent Bastard Tongue: The Untold History of English, an intriguing and accessible book by John H. McWhorter, questions this conclusion. Or at the very least complicates it.
German, Dutch, Swedish, and the gang are, by and large, variations on what happened to Proto-Germanic as it morphed along over three thousand years. They are ordinary rolls of the dice. English, however, is kinky. It has a predilection for dressing up as Welsh on lonely nights.
Its resemblances to Celtic languages like Welsh and Cornish include both the way it uses "do" to construct verbal phrases and the way that gerund verbs are used in sentences. In these English is unlike any other Germanic language. Or nearly any other language at all, with the exception of the Celtic ones.
To be sure there could be more debate. McWhorter does, however, point out that only a couple hundred thousand from the Anglo-Saxon tribes invaded Britain. Not enough to kill everyone and replace their language full-on, even assuming that's what they wanted to do.