You have probably noticed the lang/english.php and lang/english_default.php files?
The files should be never be edited directly; they should be edited using the Translation / Editing Tool that is supplied in the Admin.
To edit the language, go to the Admin and click on "language" from the menu. Then click on the "Translation / Editing Tool" link. You will be given further instructions there.
To create a new language, first create a blank file with the name of your language. For example, french.php and upload to the lang/ directory. Give the file write permissions (chmod 666)
Then go to the Admin and click on "language" form the menu. Hit the 'New Language' button fill in the details. Hit 'Submit' so save.