Hi everyone !
Thank you for your answers. I saw the Nxosc software, it looks like a good idea for a quality headtracker, it would be great with Cubase/Nuendo’s ambisonics workflow.
Unfortunately we still have that OSC communication problem (and also unfortunatly, I think that Nxosc is only developped for Apple’s ecosystem)
Sycraft-w brought us a super usefull tips in our quest for OSC headtracking in Cubase/Nuendo 
I also checked my port with TCPView and the 7000 UDP port is indeed activated when I press the “Tracking” button of the head tracking window with OSC Server device (and it’s desactivated when I uncheck the button)
So now that we know what port Nuendo/Cubase is listening to, we “just” have to find what OSC messages they’re listening to.
I asked Steinberg’s support this very specific question : What are the OSC adresses that Nuendo’s Head Tracking’s OSC Server is listening to on the UDP Port 7000 ?
But they only responded me by sending these links :
Head Tracking Window
Playback of Ambisonics Audio
I think that we are on a good way to solve this mystery !
(If by any chance a developper was passing by this thread, it would be great if he/she could enlighten us on this specific aspect 
)