This is not possible and it should not be possible.
Why is not possible:
The sku is kept in the
catalog_product_entity table. This is the main table for products. This means it's not an EAV attribute so it cannot have multiple values for the same product.
There is also an attribute called
sku defined as
static, but that one is there just for consistency. Even if you change the scope of this attribute it won't work.
Why you skould not do it:
SKU = Stock Keeping Unit.
SKU is a unique identifier for a product.
It should / does not depend on the language you are selling the product in. It should not really have meaning to the customer. Just for website owners.
If you want different SKUs for different websites you will have to create different products and assign them only in the website / store view you want visible.