I'd like to batch create users using this API and set each users language. How to do this? Is it possible to use API:Options for setting someone else's options?
Topic on API talk:Account creation
If you creating a user while not being logged in to your own account, the language will be set based on the language of the request. Use the uselang parameter to specify that.
If you're creating a user while being logged in to your own account, there's no way to do this in a standard MediaWiki installation. On your own wiki you could write a SecondaryAuthenticationProvider to provide an option for setting the language when creating the user.
No, it's not possible to use API:Options to set the options of a user other than the currently logged-in user.