Drupal 7 - Hook alters stock display message. Formatter for Stock must be set as "Unformatted" on Product > Manage display.

Drupal 7 Commerce stock - custom stock message

/**
 * Implements hook_commerce_product_reference_default_delta_alter().
 */
function test_module_commerce_product_reference_default_delta_alter(&$delta, $products) {
  $test = 123;

  foreach ($products as $product) {
    if ($product->commerce_stock['und'][0]['value'] > 0){
      $products[1]->commerce_stock['und'][0]['value'] = "On stock";
    }else {
      $product->commerce_stock['und'][0]['value'] = "Out of stock";
    }
  }
}