From your Dashboard click on the Account link in the top right hand corner.
On your account page, click the "Create a new store" button in the top right hand corner.
Create a new store in the second language you would like your store to be in. This will be a totally new store, so you can customize which products you'd like to sell, product descriptions, the default currency, etc.
Next to where you can select the language (Stystem -> Settings -> Local) you will see a "Use Multi-Lingual" link. Click on that, and then click the "Use Multi-Lingual" button at the bottom of the window that pops up. Make sure to save these settings.
There will now be a language option on your store front next to the currency option. When your customer chooses the language they'd like to use, they will be taken to the corresponding site.
Note: This feature is only available to Professional stores and above. If you are currently using a Basic or Advanced account, you will need to upgrade to access this feature.