Rooting around in the dust of history like an over-age Pigpen is what I do for a living, and also for fun. I highly recommend the study of history: it stands your preconceptions on their heads and then kicks them over, and you find some really good stuff hidden under all the accretions of rubbish. Every layer you sift raises the possibility that today’s proven facts may be completely changed by the evidence you uncover.
Looking through the history of manga for a current project, I found helpful signposts from those who had gone before. (Another huge benefit of history: it teaches us that all our achievement rests on the hard work of others. All politicians should be forced to study history, both to learn humility and in the hope that they’d stop repeating the silliest bits.)
Frederik L. Schodt is the patron saint and guardian angel of all English-language manga scholars for his seminal Manga! Manga! The World of Japanese Comics. Commenting on this excellent article on the Same Hat! blog about some translations published in Japan in 1971, he mentions that manga were translated into English and other languages for the Japanese market in the 19th and early 20th centuries. He translated the first manga actually written and published in America, Manga Yonin Shosei (The Four Students Manga, now published as The Four Immigrants Manga.) Star*Reach magazine published translated manga in 1977.
But as far as I can find out, the very first manga translated and published in the West appeared in France in 1970. As this Canalblog post demonstrates with copious illustrations, Budo Magazine Europe ran manga in French from January 1970 to October 1973.
If you find anything earlier, please let me know.