Do the translation tags also work in the Questions? Unfortunately not.

Is it possible to display PDFs in the correct language in the book? Yes

MoodleActivity / BOOK / Show Pdf in correct language

Here is the text with the language tags to copy:

{mlang de}pdf in de{mlang}{mlang en}pdf in en{mlang}
{mlang de}pdf in deutsch{mlang}{mlang en}pdf in englisch{mlang}