is there an English translation of the book by Guy Barles, "Solutions de viscosite des equations de Hamiltion-Jacobi"? Springer-Verlag
It's extremely unlikely that this 1994 Springer book has been formally translated into English, since neither Springer's site nor MathSciNet lists a translation. In fact it's uncommon for most French mathematics books to be translated "officially" into English, a subject which has come up in many other posts on MO. By various historical accidents, the most common languages used internationally in mathematics are English, French, German, Russian (with English becoming most standard as has happened in areas like trade and aviation). Naturally there are some research papers and even books in those languages which get translated unofficially into other languages and circulate privately. But French is usually considered manageable by advanced students who already know English. Translating advanced books into any language is seldom an attractive commercial venture, even for big publishers like Springer.