The 21-year-old is reportedly set to sign a four-year deal with Rennes.
Mandjeck spent last season on loan at Kaiserslautern and is not keen to return to the club.
Mandjeck said: “I am delighted to go and play in the French league.
“I played last season with Kaiserslautern, but I decided not to prolong my career in Germany. In France I will have a better opportunity to do well in my career.
“However, I have not signed anything yet and I am concentrating on having a good World Cup and helping my country to qualify for the second round.”