UPDATE 2010-05-10
As everybody seems to wait already for the latest and greatest update for N900 - the mysterious PR1.2 firmware release, I've decided to finally get prepared for that. Kudos again to firstsan, who merged the changes from the upcoming release to the currently localized files. As of version r1433 of the package, the Bulgarian localization should be PR1.2 compatible (I'm not quite sure that it will be backward compatible with PR1.1.1 though) and fully translated.
Another news related to the PR1.2 release is that this bug is marked as resolved in that firmware release. This means that we'll be able to ship quite smaller localization package, compared to the current one which includes one additional big localization related file, needed to workaround the problem.

And now - just to tease you a little bit and make a big speculation - during the translation of the newly added strings that should be used in the apps in PR1.2 there was this one:

"New MMS"

... Having said that - happy waiting for the new firmware!