Translate custom values via child theme

Hello,

I use the code snippet below to show some information in checkout when a subscription is added. Work fine. However, I have some problems with the translation. I use Loco Translate to translate plugins etc.

function eg_show_product_removed_message( $url ) {
	global $woocommerce, $eg_set_product_removed_message;

	if ( isset( $eg_set_product_removed_message ) && is_numeric( $eg_set_product_removed_message ) ) {
		wc_add_notice( sprintf( _n( '%s product has been removed from your cart. Products and subscriptions can not be purchased at the same time.', '%s products have been removed from your cart. Products and subscriptions can not be purchased at the same time.', $eg_set_product_removed_message, 'woocommerce-subscriptions' ), $eg_set_product_removed_message ), 'error' );
	}
 
	return $url;
}
add_filter( 'add_to_cart_redirect', 'eg_show_product_removed_message', 11, 1 );

Now, I added the snipped in function.php and want to translate the new added string via loco translate. I use the plugin “woocommerce-subscriptions” to translate that. But this does not work. I already contacted the Loco Translate developer.

His answer:

To do this properly you should either make your code into a plugin with its own text domain, or create a child theme with your custom translations.

Now, how can I translate such stuff via GP child theme? I already use a child theme but have no idea how to use that for translations.

Cheers