<script>
add_filter( 'woocommerce_product_single_add_to_cart_text', 'woo_custom_cart_button_text' );    // 2.1 +
 
function woo_custom_cart_button_text() {
 if( x_is_product() ){
        return __( 'My Button Text', 'woocommerce' );
 }
}
</script>