Currently I can only read with Chinese and English. So if his works are originally wrote in French, I may choose Chinese version.
According to Selina Hastings' The Secret Lives of Somerset Maugham: A Biography, Maugham's
first extant letter, written at the age of six and addressed to his parents, is in French: cher papa, chere maman, votre petit willie est heureux au jour de noel de vous exprimer ses meilleurs souhaits, et sa reconnaissante affection, croyez-moi, cher papa, chere maman, votre fils respectueux, willie maugham
However, after the death of both his parents, he was sent to England at the age of eight, so his education continued in English. My searches in The Secret Lives of Somerset Maugham: A Biography by Selina Hastings, Somerset Maugham: A Life by Jeffrey Meyers and A William Somerset Maugham Encyclopedia by Samuel J. Rogal did not find any works he might have written in French instead of English.
Though Maugham could not write French as well as English and rarely used French expressions in his letters or works, he'd been fluent in the language since childhood. He thought it was easier than English, for the vocabulary was smaller, the structure relatively simple, the grammatical rules clear. If you followed them, you could write good French.
So it appears he wrote in English because he knew that language better.