How do you set up a language pack to take effect on system messages?

Question

I'm currently finalizing the setup of a multi-lingual site and everything to this point has benefited from variables handling language-specific items, but I do have a Safecracker form in the site that automatically serves up general error messages when, for example, a field has been left blank when it's required. I have the french language pack installed, but I'm somehow missing how I enable it so that when the site is being accessed via the french URL that the error messages served back by the Safecracker form also appear in French.

In index.php, I have the following (mirrored for english as well):

UPDATED CONFIG INFO BELOW

case "french.domain.com":
    $assign_to_config['site_url'] = 'https://french.domain.com';
    $assign_to_config['language'] = 'french';
    $assign_to_config['global_vars'] = array(
        "user_language" => "fr",
        "user_locale" => "en_US.UTF-8",
        "user_locale_date_format" => "%b %e %Y",
        "user_locale_event_date_format" => "%b %e %Y",
        "swap_site_label" => "English",
        "swap_site_url" => "https://english.cps.ca",
        "country_code" => "fr",
);

I just need a bit of help sorting out this final step.

Answer 1

You may also need to add:

 $assign_to_config['deft_lang'] = 'french';  

Answer 2

I think:

$assign_to_config['language'] = 'french';

would work. Where variable value will be the language folder name as for the "system/expressionengine/language/french" the language variable value will be 'french'