If you are a Time Tracker user then the language is defined for your entire group by a group manager or a co-manager. They do it on the Group tab.
Note: due to ongoing development language files get out of sync quickly. As a result, some strings may be displayed in English (not yet translated). You are welcome to submit an improvement to a translation to your language. Changing language in Time Tracker
If you are installing Time Tracker you can adjust the LANG_DEFAULT value in config.php file. This value defines which language is used on the login page in Time Tracker.
// Default language of the application.
// Possible values: en, fr, nl, etc. Empty string means the language is defined by user browser.
//
define('LANG_DEFAULT', '');