// On functions.php
// Add to cart text on product archives (Collection) page
add_filter( 'woocommerce_product_add_to_cart_text', 'woocommerce_custom_product_add_to_cart_text' );  
function woocommerce_custom_product_add_to_cart_text() {
    return __( 'Comprar', 'woocommerce' );
}