I didn't realize this was a translation! (Marie Curie was French/Polish, I could see Einstein writing to her in English.)
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.
I don't know what languages Curie spoke, but there would have been a strong incentive for European scientists in the 19th and 20th centuries to learn to read German. That would have opened up access to a huge amount of technical literature that was being published in German.
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.
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.