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