Ok. I've tested it in English (US) and after in french. I thought this option didn't work but I found the problem.
Sometimes, I've to press the home button several times to get it working. I don't know really why, but sometimes, one long press on the home button is enough, sometimes it's not.
I tried to change the delay of the long press but unfortunately I got the same result.
And finally, I confirm that after a while, the buttons in the google app to enable search screen become grey and I must go to the phone settings to declare Google app as Assist and voice input app again. After that, no need to go to the Google app settings, I can use the search screen feature again.