Which part are you referring to?
If audio, yeah, like I said there's a way to make it so you only download one language depot, I think the only option I have seen used there is to set it to only download the language depot that matches the users Steam language. Not sure if other options also exist.
If Deck, there's definitely some stuff on the API side where devs can "detect" steamdeck, but in there I'm also not sure what their options are beyond setting bunch of default settings.
Like just doing an installer similar to the old classic regular desktop application installers.
Setting up textures for example, you would put all your different quality textures in their own depots and packages, and on install you would prompt the user to select one or more of the options. Same with the audio stuff, instead of picking a depot that most closely matches your steam language, just ask the user which one/s they want.
Maybe it's all already possible and no one uses it.
