For reference, I've found one language definition archive file from Yannick Duchêne (*) from 2011.
I'm joining the main file which I called Csound.lang instead of the original CSound.lang.
Works well, but only for csd files (i.e no orc, sco, udo...)

(*) http://www.les-ziboux.rasama.org/download/csound-for-gtksourceview.tar.gz

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