there is a directory called "language" and within there you find subdirectories for en_us, for example.
they contain the language file.
just translate them:
'register' => 'Register',
change the thing after the arrow (=>) to the word in your language.
and of course keep the " ' "
in the directory above you can just copy on of the files corresponding to your language, eg en_us.php to de_de.php (for german)
if you want to test your translation, go to config/config.php and change the lanuage setting ( define('DEFAULT_LOCALIZATION', 'de_de'); )
to your language
that's it