His words came after the French foreign ministry made similar calls for the leftist guerrilla group to live up to a February 26 promise to release hostages and abandon the practice of kidnapping.
Santos said he had “clear indications” that the FARC were holding Langlois, newspaper El Tiempo reported.
“The FARC promised the country and the world a few weeks ago they would stop kidnapping,” said Santos, according to the paper.
“The whole world and Colombia will be watching whether the FARC complies with its word,” he added.
The president reiterated calls for Langlois to be released, emphasising reports that the journalist was injured before being taken hostage.
Officials with the International Committee of the Red Cross were trying to confirm Langlois was being held by the FARC, according to media reports.
Langlois was reporting for news agency France 24 on Colombian military counter-narcotics operations in southern Caqueta Department when the group came under fire from the FARC.
Anncol, a leftist news organization with alleged ties to the FARC, reported Langlois was part of a plan to film a military attack on the FARC.
Langlois was reportedly shot in the arm, before he removed a military-style helmet and vest and approached the alleged FARC fighters.
France 24 has sent two journalists to the country to investigate his disappearance.
Langlois’s kidnapping has sparked a debate over journalists wearing military protective gear, the paper El Pias reported.
“Soldiers are soldiers, those who are civilians, including leaders and journalists, we should not take military uniforms,” the paper quoted Colombian vice president Angelino Garzon as saying.
Garzon questioned the practise of embedded journalism and whether the military should allow media to accompany operations in conflict areas.