I seem to fail at googling a simple question: In which language did Pólya originally write “How To Solve It” (or “Schule des Denkens”, as is the German title)? Did he translate the book himself to other languages?
How To Solve It was originally published in English in 1945 by Princeton University Press in English, after being rejected by three other U.S. publishers.
However, the original text at least started out in German as a draft. Pólya began writing the draft prior to 1940, while he was living in Zürich, and presumably initially intended for the text to be published in Germany. However, the political situation in Europe at the time forced him to relocate to the U.S. Naturally, there weren't many opportunities in the U.S. to publish a book in German, so he published it in English instead. The English version was substantially different from the German draft, not solely a translation.
Here is a quote from an interview with Pólya saying the same thing, reproduced from :
MP: What I would like to ask about that, then, is: When did you develop an interest in teaching at the earlier levels and was this in any way related to your work with secondary school teachers in the teacher institute programs?
Pólya: No, I was interested in it fairly early. I have also a teaching certificate, as I told you, for the lower classes, in Latin and Hungarian. In fact I have a teaching certificate for all classes of the gymnasium in mathematics, physics, and even philosophy, and in order to get the certificate you must be a practice teacher. So I taught in the high-school level. They call it gymnasium, for kids between ten and eighteen, but it is different in several respects from a high school. I was a practice teacher for a year. So I had experience. I must tell you, How to Solve It was written really twice. I wrote something, a draft, in German while I was still in Zürich. Then I came to America and in this respect, my coming to America was, I think, useful, because here, in this country, there is more interest in the “How to” books. And, by the way, Hardy predicted it to me. When I told him about the “How to” book, he said, “Oh, you must go to America.” And then I rewrote it, it appeared in final form in English. It is considerably different from my original German version, and I think to its advantage.
: D J Albers and G L Alexanderson (eds.), 2008. Mathematical People: Profiles and Interviews (2nd ed.) (pp. 257-258). p. Wellesley, MA: A K Peters.