Fix t3_sourceLang in exported xml files of l10nmgr

The value of t3_sourceLang was in any case taken from export configuration. If you override the source language on export (via backend module or via cli) value of t3_sourceLang was still taken from the configuration. 

Authored by Marko Milojevic

git-svn-id: https://svn.typo3.org/TYPO3v4/Extensions/l10nmgr/branches/v4@85387 735d13b6-9817-0410-8766-e36946ffe9aa