First i made a sync for product form elements including attributes. So if a product node got changed all the translations got updated aswell. (i can imagine this could be done easier with replacing the node values with the tnid values on the node load)
Next i rewrote the mysql queries for the shop parts my customer has access too. Like reports, product overview etc. I made the queries look for the SKU instead of the nid.
Also i had to hook into the cart for the correct translations when someone switches languages and has products in the cart.
Really ugly but it had to be done at the time.