Edward von Hohenstaufen said:
In my patch I renamed them "Volgabulgarskie"
That's a (Slavic) plural adjective form. If you want the OCS translation for "Volga Bulgar mercenaries", it would be "Volgobulgarskie naimniki" (respectively, "Volgobulgarskii naimnik" for the singular form). If you want just "Volga Bulgars", then it would be "Volzhskie bulgari" ("Volzhskih bulgar"; though you can also change places - "Bulgar(i) volzhskih/e")). And if you want their "native" language form, then the best would be to use the Arabic form "Bulgar" (or "Burdjan", though that was used for the Danubian ones, AFAIK), though I don't know what their sl and pl forms are (as well as their word for "mercenary", of course).
(Btw, the "Volga-" isn't absolutely necessary - the Russians often use "bulgar-" for the Volga Bulgars and "bolgar-" for the Danubian ones, at least today).