If you want to add new language, just click "Add new" in the top-right corner of table.
Name - Name of your language
Language culture - The language specific culture code
Unique SEO code - The unique two letter SEO code. It's used to generate URLs like - http://www.yourstore.com/en/.
Flag image file name - Flag image file name of your language
Right-to-left - Is your language supports RTL?
Default currency - Default currency to your language
Published - Publish language or not
Display order - Display order of your store
You can also export/import resource strings from/to XML files.
If you want to change the specified parts of store, look for them at "String resources" tab. Just search for the specified word like "Wishlist" and you will see all resource strings which contain word "wishlist", just change it as you want.