The xosd library bugs when you try to put a word with accentuated characters or other UTF8 characters.

If you type for example :

myOSD = Xosd.new(1)
myOSD.puts "Démarrage"

You see only "D" on your screen

I tried to look at the source file, but as I am a very beginner in Ruby, I don't understand anything...

Anyway, if you have a 

require 'readline'
input = Readline::readline()

before the accentuated text, everything is printed properly. (Same behaviour with Asian characters...)