The 34-year-old says the time is right to move to "a less competitive league" and has turned down a new deal from the Gelsenkirchen club.
"It was anything but an easy decision for me. I would like to thank the club, the president, the manager - they were all very patient with me.
"I would like to say that my future is going to be outside of Europe.
"I will sit down with my agent once again and we will analyse the offers, but it will be a less competitive league."