The issue lays in character encoding. i-doit is currently using ISO 8895-15. In short, this encoding is limited to Western European languages. The result is: If you want to translate i-doit into another language which is not supported by ISO 8859-15 it'll become a problem. You have to prepare the whole frontend and backend to work with an encoding standard like Unicode/UTF-8 which supports many more characters for almost every language. The preparation affects the source code, all translations, HTML output, AJAX requests, and PHP itself. Only the MySQL database backend uses UTF-8.
Look for any encoding/decoding methods in the source code and patch them. For example, take a look at all files which contain "8859" (index.php, ajax.js, js.php, and functions.inc.php). All files which contain non-ASCII characters have to be converted into UTF-8 (especially the language files under src/lang/). I can't tell for sure if these changes will perfectly work without errors. I haven't tested it yet. But give it a try!
