Currently, you can't get the list of group chats and multi-person chats that your LINE Official Account is participating in.
Currently, there is no function to switch the language of messages or rich menus based on the user's language.
Use the Get profile endpoint to get the user's language, and then send messages or set up a per-user rich menu based on the user's language.
The order in which messages are displayed depends on how the messages are sent.
If endpoints used for sending messages are executed multiple times, the order in which the endpoints are executed and the order in which the messages will be displayed won't be necessarily the same.
If an endpoint for sending messages is executed once and multiple messages are specified in the request body, the messages will be displayed in the order specified in the request body.