Hi,
After a whole day of testing and searching for a solution, here is my case and solution.

Environment
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- Debian Linux
- Apache 2.2.9
- PHP 5.2.6
- gettext 0.17 (faster than php_gettext )
- locales en_US

Issue
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The main issue was, C5 was not accepting any localization I would use except the default (English).

Solution
========
I used the following command to add the needed locales (ar_XX for Arabic) and restart Apache:
Also, I used ar_KW.UTF-8 for my LOCALE const. Once done, refreshing my browser showed the correct localization, without having to clear the cache.
Also, when I update the translation and regenerate the messages.mo file, I don't have to clear the cache either (browser or local filesystem).