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.
It’s the syntax that baffles me.
And the character alphabets versus pictograms like Chinese or/and Japanese.