Language is such a fluid thing.
I wonder why there are so many?
The Indo-European languages have been the most studied--extremely diverse now but all going back to a common ancestor or closely-related dialects five to six thousand years ago. David Anthony's The Horse, the Wheel, and Language has a lot of fascinating information on the rise and dispersal of the Indo-European languages.
English and German are from the same branch so more closely related than say, English and Kurdish or English and Russian, but German is still difficult for a native English-speaker to learn. German has shifted some consonants compared to the ancestral forms: German has D where English has TH (denken/think, du/thou, etc.), B where English has V (Knabe/knave, sieben/seven, etc.), and other cases.
Sometimes there is also a shift in meaning: German sterben "to die" vs. English starve, German Herbst "fall" vs. English harvest, German Tier "wild animal" or "beast" vs. English deer, etc.