language subdomains

Have you considered extending the url_helper class:

https://www.codeigniter.com/user_guide/g...lpers.html

And replace the redirect to accommodate your subdomain redirects?

There's no such thing as an "url_helper class". It's not a class, just a bunch of global functions declared by the same php file.
To "extend" them means to completely replace them.

Sigh... my bad, the link covered it with this note:
The term “extend” is used loosely since Helper functions are procedural and discrete and cannot be extended in the traditional programmatic sense. Under the hood, this gives you the ability to add to or or to replace the functions a Helper provides.
I assumed the OP can read.

to the OP each function in the helper is wrapped with:

PHP Code:

if ( ! function_exists('redirect')) 


creating the MY_url_helper.php file in the application/helpers folder with the single re-written redirect function to handle the subdomain redirect would work.