Change options strings for subscription products (works for All Products sub options)

change_strings.php

<?php
function change_text( $subscription_string, $subscription_details ) {
  if ($subscription_string == '<span class="subscription-details">every 2 weeks</span>') {
    $subscription_string = '<span class="subscription-details">Fortnightly</span>';
  }
  if ($subscription_string == '<span class="subscription-details">every month</span>') {
    $subscription_string = '<span class="subscription-details">Monthly</span>';
  }

  return $subscription_string;
}
add_filter( 'woocommerce_subscriptions_product_price_string', 'change_text', 10, 2 );