[Frankenstein]

Like most apps, OmniPlan picks its default language from the topmost language in the Language tab of System Preferences' International section. To get the French interface, either drag "French" to the top of that list or ctrl-/right-click the OmniPlan application in the Finder and choose "Get Info" from the contextual menu. You can then expand the language section in the info window and disable any languages other than French. Do this only after you've quit OmniPlan, though.