1.If you are a new user and haven't uploaded any products, just go the "Settings"->"Language" to add your language and set it as default language.
2.If some products and categories have been added to your store and now you want to change the language for your store, there are several steps.
Step 1: Add your desired language.
Step 2: Re-edit all your products, categories and save. (Otherwise, your products and categories won't show up in your new language.)
Step 3: Set your new language as the default language.
Your store is ready now!