All text can be found at one central place - the language files. For each language one file resides in the folder /language/.
You can edit the files using a text editor.
Carry out following step sif you want to create a new language file,
Copy one of the existing language files and change the language abbreviation.
Open the new language file with a file editor and make your changes.
Open the file /configuration/default.inc.php and look for the option
Add your language to the array:
$c5t['available_languages'] = array(
'en' => 'English',
'de' => 'German',
'es' => 'Spanish',
The new language should now appear in the admin area -> general settings.
Please note: You can use the same procedure for the language files inside the folder /language/utf8/, except that you need to save the files formatted as UTF-8 without BOM. If your editor only offers the option UTF-8, you need to open the language file as ASCII and remove the characters that are at the top of the file in front of <?php.
Once you have finished the translation of a file you could support us by sending in that file so that we can add it to the official version of the script.