functions.php

<?php

/**
 * Add to card text
 *
 * @param  string $text Default text.
 *
 * @return string       New text.
 */
function cs_add_to_cart_text( $text ) {
    return __( 'Add to cart', 'woocommerce' );
}

add_filter( 'variable_add_to_cart_text', 'cs_add_to_cart_text' );
add_filter( 'grouped_add_to_cart_text', 'cs_add_to_cart_text' );
add_filter( 'external_add_to_cart_text', 'cs_add_to_cart_text' );
add_filter( 'add_to_cart_text', 'cs_add_to_cart_text' );
add_filter( 'not_purchasable_text', 'cs_add_to_cart_text' );