Pierre-Luc: on the product catalog side: everything you have in your PW arsenal, will be in your PadLoper shop also. So multi-language. image galleries etc.. all that kind of things will be done how you like to do them with ProcessWire. So multi-language products are definitely easy thing.
Your idea about multilang emails is interesting - and I have it already covered. I actually don't store any templates in text files (I dislike any templates in text areas). What I have setup is very simple (but may I say clever!) thing.
All the default templates are stored in /site/modules/PadLoper/templates/*.*. Those are normal PW template files. so you have all the template variables and multilang stuff available there. But I don't want anyone to edit those files directly, so you can add same templates into /site/templates/padloper/*.* and PadLoper will use those instead.
So let's say we have email-order-confirmation.php template file. It's by default multilang ready, but if you want to customize it further, you simple copy the file into /site/templates/padloper/email-order-confirmation.php and edit it to your needs. And remember, you have all the template variables available, like $pages, $modules, $config etc.. Many of the templates also get $order variable, which has the current order and all it's information (customer, products, prices, taxes etc).
If you want to use these templates in your own code, it's also simple:
$t = $modules->get("PadRender")->getPadTemplate("email-order-confirmation.php");
$t->set("order", $order); // This template file requires $order variable
echo $t->render();
That make's it sure that you always get the customized version, if you have one, and the default one for those that don't have.
And then of course I have shortcuts for the most common templates, renderCart() etc - but customizing their markup is as simple as others (just creating copy of the file into site/templates/padloper/ folder.