I followed this tutorial http://mel.melaxis.com/devblog/2005/08/06/localizing-php-web-sites-using-gettext/ where Hello World is translated into German. If I want to translate Hello World to other language. Do I have to create another message.po and messge.mo file or I can add another translation in the same file or there's another way?
Normally you create one .po file per language, and split them up in different directories based on the locale name (like locale/de_DE/ in the tutorial), then you just use the specific file for the chosen locale as text domain in your implementation.