This is why English is such a pathetic language. Many languages have the same word for both (tortue, Schildkröte, tortuga, tartaruga…). So if you put the animal in the water and it says “Mais je suis une tortue, abruti”, you can ignore it and keep your smug smile (see panel 1).
Can’t comment on the cause, but I’ve found a workaround for this by launching steam from the terminal. I suppose that the gnome launcher calls a different version or uses some specific parameters.
I’m not French. You’re French.