You can check if a PO file is valid with the following command:

        msgfmt -c -o /dev/null <po file>
      

The following errors were found in pymupdf 's PO files:

gettext: docs/locales/ja/LC_MESSAGES/page.po:5802: duplicate message definition...
docs/locales/ja/LC_MESSAGES/page.po:1279: ...this is the location of the first definition
msgfmt: found 1 fatal error
      

Please ask the translator (identified by the Last-Translator field in the PO file), the language team (identified by the Language-Team field) or debian-i18n for a fix.