Thank you very much Vice-versa.
For the Aura HD I bugged several shop assistants. They usually could check the database to tell me that the Aura was registered (only in black at that time) but not in stock and the expected price. Nothing more.
I think I went again two days before its release and I got the same information, so even them didn't know when it was coming.
When asking Kobo_fr on Twitter, the answer is usually to check with FNAC.
In short I guess we'll know when it will be released the day of the release