(Credits to Jonno whose answer led me to the solution)
Allez dans systray and open Language preferences. Click Options for each language that has an extra kayboard that you want to remove.
In Keyboards section click Add a keyboard et sélectionnez a keyboard that appears in your systray and which you want to remove. This adds a new input option for the selected language. After you add the keyboard, click it and cliquez sur a Remove button. Après cela, the keyboard should disappear also from systray.
(Notez que I did not see a keyboard which I want remove in Keyboards section thus I had to add it first to be able to remove it afterwards.)
MODIFICATION : HelpfulProgrammers's answer helped me to prevent disposition du claviers from magically appearing whenever I connected to my PC via remote desktop (which they regularly did).
