All the language translation files are located in the locale directory. The translations for each language are contained in one for each language. The names of the files are based on the ISO 639 language code and the ISO 3166 country code.

The current language files are:
en_US.php = english (US)
es_ES = spanish
de_DE = german
fr_FR = french
nl_NL.php = dutch
no_NB = norwegian
pl_PL = polish
pt_BR = portuguese (Brazil)

Each of the individual translations in indexed by the english word of phrase, so in the english file (en_US.php), the line for the translation for “hello” would be:

So, to change any of the translation you just need to edit the stuff after the equal sign. Just make sure that your new translation is in quotes and that the line ends with a semi-colon.

Caution: UTF-8 encoding
The language files are all encoded with UTF-8 character encoding. This encoding needs to be preserved or any special characters will not display properly on your website. Make sure that you are using an editor that will preserve the encoding. Notepad under Windows should work just fine (if in doubt use Save As rather than Save so you can check the encoding method).