It works wonderfully and it's pretty easy (your language must be supported by the game, you can check that on the store page).
Guide to Change the Language
- Close steam completely.
- Go to Steam > SteamApps (if you have games in several Drives go to the drive you installed Red Dead Online).
- Look for "appmanifest_1404210.acf", and save it somewhere (you have to move the file just in case you can't edit it directly from the steam folder, remember to copy it back after editing it).
- Open it using a .txt editor (notepad).
- On the bottom line (next to "language") you will see "english" . Instead of english you have to write the language you want to play the game with (for example: "spanish" "deutsh" "french" "latam" and same for the other supported languages).
- Open steam again and launch the game.
Note: You can find the list of supported languagues below.
List of languages