woocommerce_product_single_add_to_cart_text.php

<?php
/**
 * Hook: woocommerce_product_single_add_to_cart_text.
 * Modify the text displayed on the 'Add to cart' button.
 */
function plugin_republic_product_single_add_to_cart_text( $text, $product ) {
  // Modify the $text here
  return $text;
}
add_filter( 'woocommerce_product_single_add_to_cart_text', 'plugin_republic_product_single_add_to_cart_text', 10, 2 );