I can change my Chromecast Ultra's language to almost any language, but only certain language selection will result in Stadia's and games' languages change. I've tested a few, both French and German work. But Chinese (both simplified and traditional), Japanese, and Korean don't work, what happens is Chromecast Ultra will pick up new language but Stadia will still be in English, and thus the game. Some games like Destiny 2 says supporting those languages on their official page but looks like there is no way for me to play it in Chinese.
@RejectHurdle Destiny 2 is a bit unique with regards to language. Even with PC or console version, it will default to the local language where you're playing the game. So let's say I'm playing the game in US, I can't use Japanese as the language, for example. I expect Destiny 2 on Stadia follow the same formula.
@Ivan Thanks, I did try another game Everspace which provides in game setting for language and it worked fine. So I guess it really depends on the game. But this is also a problem of Stadia to me to have some games language setting tight to system. Based on my experience games on Steam follow a more consistent and flexible interface of choosing language setting that you can pick within Steam.
Do you have any suggestion on how to find out a game's language compatibility on Stadia before purchasing? I want to buy Valhalla and Cyberpunk but would like to know what experience to expect.
@Ivan And also the other question I had is which language does Stadia supports anyway, I mean language for the Stadia menu interface. Based on my testing Chinese, Korean and Japanese are not supported.
@RejectHurdle Yes it depends on the game. Most games include language setting inside the game itself. I don't really know with regards to games on Stadia, since it depends on each publishers to decide which languages that's included in the game. My guess is it will be similar to the language settings on the PC and/or console version of the game. Otherwise, you can purchase the game, and if it's a dealbreaker not to have the language you want, you can return the game within 2 hours and 14 days.
As for your other question, you can check from the following article
https://support.google.com/stadia/answer/9338946
Can I change the language of the Stadia Platform?
For web and cast (living room) experiences, your Stadia language will be the same as on your Google Account. For mobile experiences, the language will reflect that of the OS on your mobile device.