@Omeg Choice of Dragon is not multilingual per se. The Spanish version is a completely separate game. If I understand correctly, OP wants to have both versions in the same game but with a language toggle of some sorts.
Also, Choice of the Dragon is not the only Spanish title because it’s short and a milestone in the company’s history, but because it was a commercial failure. Had that been successful, they would have released more titles in Spanish (and perhaps even other languages), even longer games.
To answer @Loud_Gecko.
CoG has, if not officially, at least through its owners and staff, made it very clear they won’t release titles in other languages due to translation costs and poor return of investment. It is unclear if that rule also applies to Hosted Games or to just their own games, and as far as I know, there haven’t been any attempts in that regard. However, consider that even Hosted Games go through a vetting process. Generally, they do not interfere with Hosted Games but they also reserve the right to not publish games with grossly offensive material or that they disagree with. They would not be able to vet a game in a different language, even if you have a version in English, they would not be able to vet the other versions.
That is not to say they would never publish a game in a different language. Like I said, as far as I know, it hasn’t even been attempted yet. Your best bet is contacting them and negociating. Maybe if you have a public beta for the version in French and a certain number of forum members approve of the content (in English) then they’d be amenable?
Games in development change a lot and updating an English version and a French version might be too much work. Even if you manage to get a green light for a French version, my advice is to wait until the English version is completed and stable, but not to translate it while it’s still undergoing revision.
Now, regarding implementation, there are some UI elements that are hard-coded in English, for example the “Next” button after a choice, and there’s not much you can do about that without tinkering ChoiceScript source code.
I would also advise against having a toggle available all the time throughout the gameplay. Make it a choice in the beginning and then the person will play that version till the end. The translated version wouldn’t be that much different from the original one, but you’ll have to take care of gender declinations and other particularities of romance languages. Page breaks should also have the display text explicit (Prochaine Page or Continuer), otherwise it defaults to English.