In order to change language in Fireboard forum, installed on Joomla, first download and copy desired language files into administrator/com_fireboard folder. You should change system language in Global Configuration. After that step, language in Fireboard should be change automatically. If this doesn't happen (e.g. in case you're using Joomla 1.5), rename your_language.php file to english.php (e.g. serbian.php => english.php)