(It would probably be in German originally if Einstein said it.)
I'm not sure about that. His first jobs were in Switzerland (where most of the educated population speaks English, among other languages of course) and he moved to the U.S. permanently around the start of the war. Also, English is (at least now) the first language of the scientific community. Einstein's famous 1939 letter to Roosevelt (on which he was evidently not first author, but nonetheless contributed to enough to sign) is in fluent, sophisticated English. There's no doubt that he was comfortable with English during a good portion of his life.
I've read several books by and about Einstein and other books in which he was a major player - and I don't recall him ever making pithy observations on human nature.
You evidently don't feel that philosophical struggles with religious questions, a topic he was deeply interested in, fall under the umbrella of "human nature."
He was incredibly focused on the nature of the universe
But not to the exclusion of other issues. Especially during the war, he was an outspoken pacifist and humanist.
Another quote often attributed to Einstein is "Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." I can't track down a definitive source for that one either, but it is at least within the realm of something he might say, for example in frustration over the development of nuclear weapons. The quote, "I do not know with what weapons World War III will be fought, but World War IV will be fought with sticks and stones" is endorsed in Einstein memorabilia publications from Princeton, where one would think they would have an incentive to get it right.
I can't tell how definitive
http://www.einstein-quotes.com is, but presumably most of the quotes are at least approximately correct, and they give an accurate impression of the wide range of insight of which Einstein was capable.