It's a trademark issue
Disney’s alteration of film and character names is not an uncommon occurrence for French versions of English films. With past movie name changes including the likes of Finding Nemo that became Le Monde de Nemo (The World of Nemo) despite the Quebec French language version remainining Trouver Nemo (Finding Nemo). However the swap from Moana to Vaiana was claimed to be due to possible trademark issues that the name posed in Europe. With Disney Spain tweeting that “The ‘Moana’ mark is registered in Spain and in some European countries. So the film ‘Moana’, will be Vaiana.”
The Monsoon Project
The reasons behind the change of name are not completely clear, but a tweet from Disney España gives an important clue: ‘Moana’ is registered as a trademark in Spain, and in a number of other European countries, hence the change to Vaiana in Europe. According to the INDAC blog, one of the creators of the cartoon, John Musker, also confirmed this information at the last Annecy Film Festival. ‘Moana’ is not legally available for use in most European countries and, given that ‘Vai’ means ‘water’ and ‘Moana’ means ‘ocean’, the sense is ultimately the same.