I believe this is the way to do these kind of things. I agree that it makes sense to provide translations in External documents (mind the word external). But I am not sure that it is always a good idea to show the customer translation to the user that is creating the Order.

The user creating the order is not necessarily fluid reading Dutch, Danish, and Russian.

So presenting the Russian word for Bicycle (Google translate: велосипед) may not make a lot of sense. So in my view the NAV Pages should show data in the users language, and the external documents presented to the customer should be in the Customers language.

A way to make a link between the user and customer language, could be to have both languages shown on the order entry eventually showing the order in a factbox in the Customers language

Rhetorically: Your are not translating the whole Order entry page to Russian just because you enter a customer with a Russian language setting, so why the Item Description!