I haven't had a chance yet to download the Feature on Demand discs from VLSC, are you saying they don't supply cabs for 2004/20H2 to add additional languages?
EDIT: I just downloaded the Feature on Demand Discs from VLSC
I see the language cab files there. My assumption is that I'll be able to slip in the other languages I need using those cabs, just like I did for 1809 and 1909. (Starting with a Base of the en-US Media)