With User support - Work Tracker (mod_tracker) installed, if you try to run Language customisation it will fail due to what gets evaluated as duplicate entries in the lang/tracker.php file. Moodle evaluates errorinvalidelementID and errorinvalidelementid as being the same. Both strings are in both of the current mod_tracker versions available for download, 2.2.0 and 2.3.0, in the Moodle Plugins Directory.