LightHeaded, EveryQuest, TourGuide are great addon but none of them provide their data in other language than English.
Instead of doing an addon that would contain all the data they need why not give addon author access to a data repository which contain all basic quest data. They can then choose which data to use and which format to store the data in their addon.
If you would like French language data, just ask Wowhead for it, or for permission to mine it and distribute it.
Or you could ask Curse/WowDB; they appear to have a French language option. (I'm unsure if they actually have French data, though.)
It's a good thing they are adding a way to retrieve completed quest history.
My only concern with this is that they are only giving quest id. If no api is provided to retrieve quest data we'll have to rely on a quest db.
would it be possible to build such db automatically from a wowhead automatic datamining? (in all available languages?)
The following information would be interesting for each quest id
start npc (or start item)
previous quest id if in a quest chain
next quest id if in a quest chain
Several addons already maintain a database of quest data. Usually they get and distribute these data with the permission from one of the major database sites (usually Wowhead).
You can take a look at EveryQuest (only basic information) or LightHeaded (includes quest starters, enders, series, rewards) for two examples of addons that can easily incorporate the new quest completed data.