We had an issue recently when trying to upgrade Exchange SP1 to SP2 , while running through the Pre-Reqs we failed on the Unified Message role due to having more than the en-us language pack present.
You can find what additional UM language Packs you have installed via running the following in the Exchange Management Shell.
Get-UmServer | fl name,languages
You can then uninstall the management pack by running the following command (Our example is en-GB)
After this you should pass that part of the Pre-Requisite check