How to localize Product and Categories descriptions

How to localize product's Name (same for descriptions or any type of content you would need)

This is handled through the alternate locale feature of the Content Manager. Each ProductContent will point to the content for the "default" locale, and that Content can have other contents associated with it as alternate locale contents. There are various reasons for this structure, but the main one is to be able to create a tree of defaults and to use a generic structure rather than one for each place Content is used in the system.
For clarification it goes like this:Product
-> ProductContent
-> Content -> DataResource
-> ContentAssoc (type: ALTERNATE_LOCALE)
-> Content -> DataResource
-> ContentAssoc (type: ALTERNATE_LOCALE)
-> Content -> DataResource
-> ContentAssoc (type: ALTERNATE_LOCALE)
-> Content -> DataResource
...

Step by step guide (for Product Name, same for descriptions or any type of content you would need):

1. In Catalog Manager, go to the Edit product content tab.
2. Clear the Product Name entry (respectively descriptions) in the "Override Simple Fields". When localizing, these fields must be empty. They have precedence over the localized product contents.
3. Create a "Product Name content" (respectively descriptions) in your preferred locale. Actually four entity values get created: ProductContent, Content, DataResource, ElectronicText.
4. Create any alternate locales as you need using the bottom part of the screen.

Now, in ecommerce, the product name (respectively descriptions) should be displayed in diffent locales depending on the browser settings.

An alternative (without UI) is to follow the content of DemoProductI18nData.xml, see also DemoProductCategoriesI18nData.xml