Nowhere any time soon, I guess, as the author is Russian.

I think the book is free because Egor and PostgresPro want to develop the community. If you want to pay back - you can just start contributing to Postgres. No need to wait :)

> The translation is currently in progress. Right now, only Part I is available. Other parts will follow soon. Stay tuned!

That doesn't answer where it can be purchased, nor does it imply that it will be available for purchase when all translations are complete. At 167 pages, part 1 could conceivably be it's own book for purchase. I too would love a resource like this as a book and would be willing to pay for it. I would even purchase each part individually as they become available.

As in it's available in another language already?

Yes, it seems to be Russian originally. See the first page of the book:

    PostgreSQL 14 Internals
    by Egor Rogov
    Translated from Russian by Liudmila Mantrova
    © Postgres Professional, 2022

It's available for free in Russian: https://edu.postgrespro.ru/postgresql_internals-14.pdf

linked from their own website: https://postgrespro.ru/education/books/internals