You forgot the best part!
"...by means of a comical witticism"
It's a pretty amusing way for Einstein to describe his derivation, anyway.
I thought it was just Einstein writing extremely strained English. If the original reads fluidly, then the translation...could use improvement. It's comically strained.
My father said that when he was a kid in the 1960s, there was still a sense in the U.S. that it would be useful to learn some German if you planned to go into fields like math or physics.
But I wonder how European scientists at the time of this letter chose what language to use in their personal correspondence.