Custom language translation

This changes allow user make own translation without overwrite vtiger language files. It's neccesary to change language directory structure into: language/modules/language/module.php file

like

language/modules/en_us/Leads.php

In this file you can add into language array your translation like: $languageStrings = array( 'LBL_ADD_RECORD' => 'Create new Leads', );

After migration into new vtiger version your translation will be not overwritten.