Hi,
I suppose that you're using Falang or JoomFish to translate your content.
In that case, the main description of the product is used for all the languages of your website by default.
And when you write a translation of that description for a language, that translation will be used instead of the main one for that language.
So if you have the main description of your products in English, you would just have to edit the translation of the products in Falang for the Dutch language and replace the main content by the translated one and save.
So I don't see the need of copying the description from one language to another in that case.
But maybe there are other details to your situation that I don't know which would make that necessary ?