Consider the singularity of Vladimir Nabokov's achievement: the Humbutterfly Hunter went from being one of the major Russian writers of the twentieth century to being an even more major English-language writer. If he had never written an English word (not a single plaintive and, not a single breathy the) his Russian novels alone would have assured his canonical status in world literature--and the same is arguably true of his English novels, if, by some strange historical accident, he had never written a Cyrillic nyet or a funny-looking da. It's a critical commonplace that Lolita, Pnin, Transparent Things, and (for some critics) even Ada are uncommonly fine, but less common, for some reason, is the idea that Despair, The Defense, Invitation to a Beheading, and The Gift can stand alongside any other Russian-language works of the grey and gloomy Soviet years--even Bulgakov's supreme The Master and Margarita and (to name a work Nabokov surely despised from the Olympian height of his brow) Doctor Zhivago. Offhand, I can think of no analogous case of a dual-language major novelist. There are multiple examples of novelists writing in 'non-native' languages: Conrad, Beckett and Kundera form a very strange troika of such double-tongued ones. But Conrad, as far as I know, wrote little of note in Polish; Beckett's English work is (probably unfairly) less highly regarded than the books he wrote originally in French; and Milan K. seems to have traded his position as a major Czech novelist for a late-life minor niche in the cathedral of French literature. Is Vlad the Inscriber's achievement unique, or is there an analogous case that's stubbornly staying out of my mind?
(A half-hour after sending out this post, a possible contender comes to mind: Ngugi wa Thiong'o, who has written major works in both English and the Gikuyu language of his native Kenya. And there are surely other examples from the formerly-colonized world...)