yii\i18n\MessageSource::loadMessages PHP Method

If translation for specific locale code such as en-US isn't found it tries more generic en.

protected loadMessages ( string $category, string $language ) : array
$category	string	the message category
$language	string	the target language
return	array	the loaded messages. The keys are original messages, and the values are translated messages.

    protected function loadMessages($category, $language)
    {
        return [];
    }

MessageSource