Does the word “would” means “was” here?

Question

Writing to the woman who would become his third wife, Hermann Hesse complained: “Life for me now holds almost no pleasures any more, in fact I am living in Hell.” The event that had reduced Hesse to this state of near-despair was that his wife-to-be Ninon Dolbin had moved some of his books without his permission. For him this was an intolerable disruption of the orderly existence he believed essential to a writer who had detached himself from the world. His independence required that he hold all of humankind, and even his closest companion, at a rigorously policed distance. Accordingly, although the two of them lived under the same roof, he communicated with Ninon mainly in writing. As his latest biographer, Gunnar Decker, relates:.......,

My question is regarding the word “would” . I am translating this into a different language and I know would in english here might mean there was a habit in his life to marry multiple time and she was the third wife. My question is how translate “would” into a different language ? Should I just say “she was actually his third wife”

Answer 1

My question is regarding the word “would” .

would is the past tense form of will. And will indicates the future.

First, go back to when he was writing the message, in the past.
From that point, he will (in the future) marry his 3rd wife.

Translation solution - the simple past may be easier:
Original sentence: Writing to the woman who would become his third wife
New sentence: Writing to the woman who became his third wife

Answer 2

This is not the habitual would (as in "now and again he would remarry"). It simply means "who was to become" - as a historian, we can look forward in time from the point where they first met as we already know they would later be married.

https://en.wikipedia.org/wiki/Future_in_the_past