I posted a link to this topic in the russian forum.
Here is the
Google translation of a partial answer by a representative of Pocketbook :
"On obtaining the source code.
If you want to receive them, please write a formal letter to the mailbox
help@pocketbook.ua
Subject: Source code.
I beg to present your request in correct form, detailing your request.
Response to the letter you receive either of the box, or you will start to carry on a conversation one of the developers."
(but do they speak english ? Maybe someone here can help to write it in russian ?)