Although a French audio track was available for my movie, in Netflix, on my mobile (Android), it was not anymore once I started to cast that movie on my chromecast.
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NB: French audio track are available here in Belgium as in France.
The really weird thing is that this issue wasn't occuring for all the movies and series. I noticed it for the serie "Merlin", season 5. At the same time it was running fine for the serie "The 4400" for example (French audio track was well available in Netflix both on my mobile and on Chromecast for those).
To fix it, I started the App Chromecast and went into the tab Devices. In the Settings of my Chromecast, I set French as language.
If the problem comes back, using the same App Chromecast, reboot the device. It's a command available in the menu of the device's Settings. Next, start the movie with the right audio track on your mobile and then cast it to your Chromecast.