English's closest living relative still can't be understood by English speakers

Frisian, spoken along the Dutch and German North Sea coast, is linguists' pick for the closest living relative to English, sharing an 'Anglo-Frisian' branch no other language belongs to. Centuries of separate borrowing from French, Dutch and German pulled the two apart, so the old Frisian proverb 'Bûter, brea en griene tsiis is goed Ingelsk en goed Frysk' sounds naggingly familiar to English ears but isn't actually understood by them.

— Wikipedia contributors, Frisian languages — Anglo-Frisian languages