The answer to this question largely depends on what you're really asking. In terms of submissions, the Festival de Cannes and the various sidebar organisations accept films on a range of formats, including common digital ones. The exact format requirements vary depending on whether the film is a feature or a short, which section you are submitting to, and in many cases, what level of submission fee you wish to pay.
When it comes to a film's shooting format, the question is somewhat moot as the majority of films are now originated digitally these days. The Festival de Cannes (and the various sidebar organisations) do not have any specific rules about shooting formats. A film will be selected based on it's overall quality as a cinematic work.
However, where a film is selected for a festival berth, a DCP or a 35mm print will need to be made available for the official screenings. The festival currently does not screen films in any other formats. The costs of preparing screening prints or DCPs must be met by the film's producer (or appropriate representative).
For shorts, only those selected for the short film competition must have a print or DCP. Shorts in the Short Film Corner are only accepted in digital format, and for Cinéfondation films, the Festival de Cannes works closely with the respective schools to arrange the best screening format for selected films.