That's interesting. Especially since as a kid, I had a "xylophone" that was metallic. That's how I learned the word and everything.
I even found this definition:
1. (Musique) Instrument de musique à percussion composé de lamelles de tailles et d’épaisseurs variables qui forment un clavier sur lequel on frappe avec une mailloche.
That doesn't specify the "wood" part at all.
And to be honest, I didn't even know
"xylophones" could be made out of wood. All the ones I've ever seen where metallic. I guess "glockenspiel" is too difficult a word for French people