To add to Gallifreyan's answer, I found an English-language interview with Elif Şafak for the Turkish newspaper Zaman, which used to be available here (although the newspaper was terminated in 2016) and can still be read here (albeit with the infuriating issue that every Ş is rendered as Þ):
You say, “Both the English and the Turkish versions of this book are original,” in one of your interviews. What do you mean by that?
Well, for the last five years I have been writing fiction in both English and Turkish. Several of my novels were originally written in English, then translated into Turkish. Several others were written in Turkish, then translated into English. So I am a writer who enjoys commuting between languages. In “The Forty Rules of Love” I tried a completely new technique. I wrote the novel in English first. Then it was translated into Turkish by an excellent translator. Then I took the translation and I rewrote it. When the Turkish version was ripe and ready, I went back to the English version and rewrote it with a new spirit. In a way I have built two parallel books in the same span of time. It is a bit insane, I have to admit. It is a crazy amount of work. I do this because language is my passion.
Summarising that: she wrote it in English first, then took someone else's Turkish translation and rewrote it in Turkish, then rewrote it in English based on that. So while it is accurate to say the book was first written in English, that's not the full story: the final English version seems to have been influenced by the Turkish version, and probably some aspects were first written in Turkish.