I'm interested to get feedback on this. For many European languages this is an easy question (example Czech/Slovak, Dutch/German, Russian/ Ukranian), but English is a bit hybrid of Romance and Germanic languages. I speak 3 languages more or less fluently: English, Czech, and Spanish. I always felt that knowing English allowed me to become proffecient in Spanish very quick, because while the basic language is different, the complex vocabulary is very similar. So once you learn the basics you're on easy street. Meanwhile, I can't understand German at all, and it is supposed to be closer related language. The Academic answer is usually Frisian, a regional Dutch language. I don't have any exposure to Dutch or Frisian, so I can't say. Still I'd be interested in people's opinions.