With a few quick changes you can enable inline translations in Magento. This will highlight every translatable portion of your site with a red box and you can directly edit those elements to say whatever you like. This method is incredibly easy, but the changes can be difficult to track and you wouldn't want to have to translate an entire site this way. Use this approach if you have just a few one-off changes to make. 

If you want to automatically change every single translatable element of your site to another language Magento offers up language packs that make it easy to do so. There are a few steps you need to follow, so pay attention...

In the Magneto Admin Panel start by setting up a store view for each different language you want to support. Generally people have their site, then a store per currency they want to support, then store views per language they want to support on that currency. If you only want to have one currecnty and one language this will be a very short step. 

Find the language pack you want to install by visiting Magento's Translation Page. You can pick from over a hundred languages so you'll probably find the one you're looking for. Once you do click on the "Select" button and you'll be taken to that language's page. Just download the "Package" and you're on to the next step. 

All language files need to be copied to the "App" folder in your server. Fire up  your favorite FTP program and copy all the files you just downloaded to their corresponding folders on your server. 

Log in to your Magento Admin Panel and navigate to System > Configuration > General > General > Locale Options > Locale and set your store view (make sure you're in the right one by selecting it in the upper left corner) to whatever language it is you just downloaded. Save your changes, flush your cache, and you're ready to go.