I don't quite understand what you mean by "transliterated abbreviations". To use your example, a transliteration of "cm" would be "цм", which is absolutely wrong. On the other hand, the unit abbreviation "cm" has its correspondence in Bulgarian (and Russian as well) and it is "см". But this is no transliteration by any means.
And don't even think about leaving the imperial units, unless they are established in the respective area. For example, pipe fittings are usually measured in inches - although the German word "zoll" is used - and not in millimeters. On the other hand, no one would know how much is a cubic foot of earth or an acre of land.
Since in Bulgaria the SI units may be written both in the latin alphabet and in cyrillic, with some preference to the latin rendering, you need just to choose one approach for the unit names and use it consistently. Of course, converting units from imperial to SI is tedious and error-prone, but leaving a table with, for example, values of torque in pound-feet is simply useless.
And I have to stress on the consistency part. Don't use one style of unit names representation in the text and the other one in the tables - assuming the units used are the same, not SI and imperial. Although the readers will probably have no trouble understanding it, at least to me seeing such a manual would be a sign of lack of attention of detail.
Actually, this question is a little strange. Just do the same what you do when translating into Italian, since Italy is "a metric country", as far as I remember. Of course, they don't write in cyrillic in Italy, but your translator(s) should know very well how to render the units of measurement in their tongue.