1417: West Germanic Mutual Intelligibility Oct 28, 2018

Ask a monolingual speaker of Dutch, German, Yiddish, Frisian, or even Afrikaans could probably understand a good amount of what a speaker of any of the other three was saying as these are all West Germanic Languages. However, when it comes to English, which is also West Germanic language, there is not that same intelligibility. The vocabulary is saturated with far more French- and Latin-based words, and even North Germanic words. However, this is not the only reason. It is generally the case that isolated peoples' languages will develop very separately, but moreover, those Germanic tribes who settled Britain did so, often before the other languages split off from each other. This is how Frisian is the closest related language to English today, and yet a Dutch speaker would have an equally difficult, perhaps even easier time understanding.