We had some customers encounter an issue with installing language packs on BitLocker protected computers. It turned out that the installation of a language pack for all users on a computer changed the boot configuration data (BCD) settings on the computer and that change was detected as a possible tampering with the computer by the trsuted platform module (TPM) which then forced the computer into recovery mode. As a result we are recommending that you suspend BitLocker before installing a language pack for all users and then resume protection so that the new settings are included in the TPM measurements.

This change is in both the FAQ and in the deployment guide in the following topics:

BitLocker FAQ entry -  What causes BitLocker to start into recovery mode when attempting to start the operating system drive?

BitLocker Deployment Guide topic - Best Practices, BitLocker Operations