By far the easiest way to add a new language to your community is to install a ready-made language pack. Many are available in the translations section of the Marketplace.
Most language packs consist of a single file with the .xml file extension. If the language pack you are installing contains other files, follow the instructions provided with the pack.
To install the pack, navigate to Customization -> Languages in the AdminCP. Click the Create New button, and in the popup window, select the Upload tab. Choose the xml file that makes up the language pack.
The Locale determines how numbers and dates are formatted in the software. The same language can be used in different locales; for example, English is used in the United States and the United Kingdom (among others), but these are different locales and use different date formats. Choose the locale that best matches your userbase (you can install the same language pack multiple times and use different locales for each, if you wanted).
Click Save to install the language pack. You'll now see it in the language list; it is enabled and ready for users to use!