It seems to me that you can compile a book from a ProofreadPage-driven page (i.e. created by means of <pages /> tag) only in English Wikisource.
I don't know why exactly, but it's not a font problem :) (I checked against Belarusian, Deutsch and Russian Wikisource, neither works.)
I didn't dig deep into the Collection code, but the only fundamental difference between English setup and Belarusian/German/Russian setup is the naming of ProofreadPage namespaces (Author, Page & Index). But all of the tested Wikisource projects (be, de, ru) have $wgNamespaceAliases set up to fall back to the English variant.
So, this is either Collection doesn't follow wgNamespaceAliases rules, or there is something way more complicated (that is why I post this bug here, and not in the Wikimedia section).
Way to reproduce (in dewikisource)
- Get a page with text from ProofreadPage extension, but not one where all icons are green - you could start e.g. from https://de.wikisource.org/wiki/Special:Random/Index
- Click 'Drucken/exportieren -> Als PDF herunterladen'
- Wait until the PDF is done.
- Download it: Dokument herunterladen
- PROFIT! You can see nothing but a title and some rubbish, but not the actual text.