QtIFW

Fix translations README file to correspond how language can be changed

Change in QTIFW-390 caused that LANG cannot be used anymore in Windows and
Linux to change the language. Instead LANGUAGE should be used in Linux and
system settings changed in Windows

Change-Id: I2fa6881652e31836d85e2b7442a39a418654ee80
Task-number: QTIFW-791
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>