What is published original source of the quotation attributed to
William the Silent of Orange, "It is not necessary to hope in order to
undertake or to succeed in order to persevere."

Researchers should take note of this statement by Martin Malia:

"Les citations françaises (Paris, 1952) and the Encyclopédie des
citations (Paris, 1959) both state that, though this aphorism is
generally attributed to [William the Silent], it is to be found
neither in his printed works nor in biographies by contemporaries."

Reply by Martin Malia to letter on "Poland's Past", April 12, 1984
The New York Review of Books
http://www.nybooks.com/articles/5880

Have a look at: http://perso.respublica.fr/chevaliersdunet/calligra.htm
It seems to me that this is not in a publication but as a motto in a letter.
I'll look it up in a (printed) Dutch history of William the Silent tomorrow.

As you will see from this and many other sites

http://www.cyberquotations.com/sorted/qUnderstanding.htm

William of Orange adopted the quote as his own 200 years after it was
first said by Charles the Bold.  I couldn't find the source of the
quote so i haven't submitted it as an answer. Since Charles the Bold
was french, typing the quotation in french just reveals more people
quoting it plageuristically without bothering to acknowldege Charles
the Bold.