For Importing Products, the best approach is
Go to System > Import/Export > Data Flow profiles
Click on Export All Products
(I assume you already know this since you have tried uploading before) Download the
relevant products into the CSV.
Fill the CSV with new information [Source: OP Referring Tutorial ]
Now In CSV file, add the relevant records for each sku you want to update/upload
Go back to Data Flow Profiles > Import All Products > upload the file, and run.
Your products will be uploaded or updated. as applicable.
For Multiple Store Views
You can use different sku if your products are different, else if you have the same inventory sku should be the same.
You need not have to change the Product sku if you want the same product with a change in attributes of STORE VIEW SCOPE
You Must Use the appropriate store view code to assign products to each store
By Default in your exported CSV, _store columns are not there, Add it and use store view code in that place.
For Just updating the Same product with a change in the Product name in a different language, you need to follow the same approach as well. You cant go to en_US or other similar files to define translation strings for each of ur products ;-)
Refer to the Similar Threads, and this tutorial Here
Importing Product for Store View
Importing Products with the ImportExport Interface