After a few days testing my custom Skill to define my thermostats, I found a problem in French version of Alexa.
I couldn't find a way set my thermostat to "ECO" mode with voice. In French "ECO" mode is pronounced "éco" or "économique" for long version. When I use these words, Alexa answer that she doesn't known how to set this parameter on this device.
To check the problem, I tried using the Alexa developer Console :
When I enter : "mets le <device name> en mode éco" I get the same answer than by voice : "Je ne sais pas comment régler l’appareil Chauffage cuisine sur ce paramètre. "
Anyway, when I try with : "mets le <device name> en mode eco" It's working!
Then I tried several pronounciation with Alexa by voice, but impossible to trigger it. (I tried "euco", "euconomique", etc, but this words don't exist in french).