form redirection

You need to determine what language the user is currently using. This is determined by whatever method you used to set the language in the first place. If it's just set by a route in your application, you need to check the relevant portion of the URI. If it's set in the config or the session, you need to retrieve it from there.

Once you know which language the user is currently using, you just send them to the appropriate page.

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide