How to set language for user register page / form per subdomain For a custom module, I need to change the language of the registration form based on the subdomain the user is currently active on. 

I tried changing the global $language variable in hook_form_user_register_form_alter, but that does not work ($language variable is also not set at that point). I also tried setting the language in the $user object, but also no dice (probably because it is an anonymous user).

Finally I took a look at the language negotiation api, but setting up negotiation providers seems overkill for this (and haven't it studied enough to get a grasp on how to use it).

I just need to render the registration page in a different (but already available) language on /user/register, based on the subdomain. What would be the best way to achieve this.

I'm using Drupal 7.
Maybe you can use the Domain Locale module. It allows you to change the available and default languages per domain.