You install the language pack. Activate it from the language manager for both site and administrator side. Then goto contents tab and create the language selection option for your whole website. Here is how it goes in details :
Goto Joomla Translations page and download the language pack, which you need.
Goto Extension Manager and install the zip file
Now goto Extension Manager > Language Manager
You must see the language which you just installed
Simply select the new language as DEFAULT for both SITE & ADMIN tabs
Goto the CONTENTS tab from the same page and click on NEW. Follow the below screenshot , how I have done it and just do the same . Make sure, the language code and the name are exactly correct , as your language file says.
Here is another very good tutorial too,
Goto Plugin Manager and you will see two plugins : Language Filter and Language Code . Just activate both of them .
Now goto your article manager and create your articles . You will see a dropdown option for language, where you can see English and the other language which you just installed and set up in the previous points . So, from here you create your contents for all the language you have installed.
Once done, you can simply activate the language filter module on the frontend of your website. You will see the languages which you installed on the frontend dropdown Simply select a language and your website will automatically the content in that language . That is how simple it is. You will need to set the language for each menu item as well, as and when required.