Making a website "international" isn't an easy task, but with Joomla's Language capabilities it's much easier to do. Joomla provides a Language Manager in the Administrator that many users probably never see. After installing the new language pack just like a normal extension, you can go to the Language Manager and set the new language as default.
Joomla also provides the ability to make language overrides for changing specific text in a website without altering the core Language file. This protects changes from being lost during Joomla core updates and provides a method of safely changing only the text you need.
Joomla language overrides are explained in this tutorial post - Language overrides from Administrator.