LimeSurvey 1.x navigating the source code
From LimeSurvey Manual
After downloading the source code via SVN (See: Accessing the Source Code) you see the following directories:
This is the current stable branch of LimeSurvey and contains the latest hotfixes and translation updates.
This is the directory with the original arts used in LimeSurvey.
In this directory are server scripts which are in use in http://www.limesurvey.org .
In this directory are the files, as they are delivered in the download package of the according version.
To translate some text displayed on screen you have to either create a new instance of 'limesurvey_lang' (limesurvey\classes\core\language.php) or in some cirumstances there is already an instance created called '$clang'.
To translate you call the function gT ($clang->gT(string)). More information on how to use localization can be found here.
In general $clang is set in (limesurvey\common.php) and uses a number of variables to determine language.