The problem:
When using hook_user_login to redirect a user to the correct language this is also triggered during password reset. And the hook params do not provide a way to deal with this.
This How can I redirect users on login based on their chosen language? post details multiple not working concepts.
And i can't comment cause of the silly reputation thing, while i'm a developer for over 10 years... (getting tired)
I guess the form alter is the ONLY way.
hook_user_login is as you'd expect for when someone goes from anonymous to authenticated. This happens by using the login form and by using a password reset link.
Having a look on Drupal's API site is always worthwhile as people have usually commented on comment occurances like this. For example on https://api.drupal.org/api/drupal/modules%21user%21user.api.php/function/hook_user_login/7.x it has exactly what you're describing and recommends using something like:
if((!isset($_POST['form_id']) || $_POST['form_id'] != 'user_pass_reset')){
//your redirect code
}
But also using form_alter would also be perfectly fine really as you want to target a specific action of logging in via a specific form
