A speaker of English and German, he could sight-read Afrikaans, Danish, Dutch, French, Finnish, Icelandic, Italian, Norwegian, Portuguese, Romanian, Spanish and Swedish.
In 1908 his mother had founded the Lawyers' and Merchants' Translation Bureau at 11 Broadway, New York (now a subsidiary of RWS Holdings).
William obtained degrees in English and Chemical Engineering from Columbia University and in Law from Fordham (1943), being called to the New York bar.
After the end of World War II, Bertsche joined the family business, and he became owner when his parents retired in 1966.
His obituary in The New York Times said he "had been one of the nation's leading document translators for more than half a century".