Dutch

Dutch is English's closest living relative among major languages. The grammar is simpler than German (two genders instead of three, no case system in practice), and roughly 80% of Dutch vocabulary has a direct English cognate. "Water" is water. "Finger" is finger. The FSI rates it at 575-600 hours. The catch: the Dutch speak such excellent English that getting them to practice Dutch with you is the hardest part. 25 million native speakers in the Netherlands, Belgium, and Suriname.