In the highly personal message, Sarkozy said Marulanda had "a rendezvous with history" and would have a crime on his hands if she died.
His brief appeal was also subtitled in Spanish.
Francois Fillon, the French prime minister, said France would be willing to take in former rebels as part of a possible deal to free Farc hostages, including Betancourt.
A spokesman for Sarkozy said at the weekend that a plane with medical supplies would be kept on standby in France ready to intervene "at any time" if a hostage deal could be reached.
The Colombian government has also offered a prisoner exchange in order to secure the release of Betancourt and other hostages.
The Farc (Revolutionary Armed Forces of Colombia) is holding hundreds of hostages as part of what it has said is a Marxist armed struggle against the Colombian government.