I looked this up when buzz was out about a Latin America data center. Basically put it's not about number of global speakers of the language, but also how many people who speak that language would be likely to play the game if it were localized into their language.

Going off just video game sales in general as well as sales from previous Final Fantasy titles, video gaming just isn't as prevalent in Spanish speaking countries as others, mainly just a thing in Mexico and Spain, and even then the JRPG genres aren't as popular. (though this could be a chicken and egg scenario where game isn't popular because not translated, leading to not translated because perceived as not popular) But either way there are likely more players speaking French playing in Canada and France than would be speaking Spanish in Mexico and Spain. Additionally there are a lot of French speakers in Africa... though I wouldn't be surprised if it's a similar situation there as in South America where video gaming just isn't as big of a thing.

German... I have no explanation for that one, except that if they were translated and localized because of the European market, German is spoken as a first language by more people on the continent than French, so it gives the game a wider market.