That still leaves the question begging: Can I translate the title as the heraldic term “Bend sinister” or do I have to stick with the literal “Under the sign of illegitimacy”?

That makes sense. After peering into my little “practical” Bulgarian dictionary, “znak za” appears to mean “sign of” and “nezakonorodenost” appears to be a compound composed of “nezakonen”, meaning “illegal”, and “roden”, meaning “born”. So literally “sign of illegitimate birth” or “bastard”. Several sources say that in heraldry “bend sinister” was often taken to indicate bastardy. Another source says that is a mistaken notion. One early Dutch translation of the novel (see D24.nl.1.1) was titled “Bastaards”. A non-Nabokov translation of the English language novel into Russian was titled “Pod znakom nezakonnorozhdennykh” (“Bend sinister”), which I think can be translated literally as “Under the sign of bastards”. There are similar title translation parallels in other languages.