This is a completely translated system at all levels. To achieve this you must define here the languages that you will be using. Probably you have recieved a lot of languages installed when you first installed the system. Feel free to delete any languages you are sure you don't want.
A very critical setting on your system is the configuration web file that sets the default language for the whole system. Shown below:
'language' => 'en',
The ISO Name is very important that you get it exactly right. Several directories are to be named the exact same name.
If you create a new language then you must create a directory as shown above and in that directory you need to copy files from a similar directory and make the appropriate translations.
If you improve on an existing translation then please send that translation to us so that others can enjoy your work.
Another important setting for language is in the user profile. Go there to define what language you want to see.