How to write a localized NTFS boot sector?
Different messages come from different places. But if you replace the boot files, like bootmgr, BCD (from a system with the exact same boot configuration), bootsect.dat, boot.sdi, or whatever combination of files that you have, I think you'll find the messages in your chosen language.
On my Vista, the boot sector is in English, even though I have Polish system. I installed from the original Microsoft disks. Perhaps your bootsector was an OEM hacked one.