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/device.po:85: end-of-line within string
docs/locales/ja/LC_MESSAGES/device.po:85:3: syntax error
docs/locales/ja/LC_MESSAGES/device.po:86: end-of-line within string
msgfmt: found 3 fatal errors
gettext: docs/locales/ja/LC_MESSAGES/document.po:1261: keyword "format" unknown
docs/locales/ja/LC_MESSAGES/document.po:1261:60: syntax error
docs/locales/ja/LC_MESSAGES/document.po:1467: end-of-line within string
docs/locales/ja/LC_MESSAGES/document.po:1468: end-of-line within string
docs/locales/ja/LC_MESSAGES/document.po:1579:115: syntax error
docs/locales/ja/LC_MESSAGES/document.po:1579: keyword "ON" unknown
docs/locales/ja/LC_MESSAGES/document.po:1814: end-of-line within string
docs/locales/ja/LC_MESSAGES/document.po:1814:3: syntax error
docs/locales/ja/LC_MESSAGES/document.po:1815: end-of-line within string
docs/locales/ja/LC_MESSAGES/document.po:2650:262: syntax error
docs/locales/ja/LC_MESSAGES/document.po:2857: end-of-line within string
docs/locales/ja/LC_MESSAGES/document.po:2857:3: syntax error
docs/locales/ja/LC_MESSAGES/document.po:2858: end-of-line within string
docs/locales/ja/LC_MESSAGES/document.po:2970:90: syntax error
docs/locales/ja/LC_MESSAGES/document.po:3319: keyword "uri" unknown
docs/locales/ja/LC_MESSAGES/document.po:3319:4: syntax error
docs/locales/ja/LC_MESSAGES/document.po:3320: keyword "title" unknown
docs/locales/ja/LC_MESSAGES/document.po:3321: keyword "to" unknown
docs/locales/ja/LC_MESSAGES/document.po:3322: keyword "filename" unknown
docs/locales/ja/LC_MESSAGES/document.po:3323: keyword "zoom" unknown
msgfmt: too many errors, aborting
gettext: docs/locales/ja/LC_MESSAGES/irect.po:306: end-of-line within string
docs/locales/ja/LC_MESSAGES/irect.po:307: end-of-line within string
msgfmt: found 2 fatal errors
gettext: docs/locales/ja/LC_MESSAGES/linkdest.po:38:2: syntax error
docs/locales/ja/LC_MESSAGES/linkdest.po:38: keyword "linkdest" unknown
msgfmt: found 2 fatal errors
gettext: docs/locales/ja/LC_MESSAGES/pixmap.po:45: end-of-line within string
docs/locales/ja/LC_MESSAGES/pixmap.po:46: end-of-line within string
docs/locales/ja/LC_MESSAGES/pixmap.po:475:101: syntax error
docs/locales/ja/LC_MESSAGES/pixmap.po:1088: end-of-line within string
docs/locales/ja/LC_MESSAGES/pixmap.po:1090: end-of-line within string
docs/locales/ja/LC_MESSAGES/pixmap.po:1166: keyword "WEBP" unknown
docs/locales/ja/LC_MESSAGES/pixmap.po:1166:135: syntax error
msgfmt: found 7 fatal errors
gettext: docs/locales/ja/LC_MESSAGES/quad.po:225:85: syntax error
msgfmt: found 1 fatal error
gettext: docs/locales/ja/LC_MESSAGES/rect.po:312: end-of-line within string
docs/locales/ja/LC_MESSAGES/rect.po:313: end-of-line within string
docs/locales/ja/LC_MESSAGES/rect.po:772: end-of-line within string
docs/locales/ja/LC_MESSAGES/rect.po:772:3: syntax error
docs/locales/ja/LC_MESSAGES/rect.po:773: end-of-line within string
msgfmt: found 5 fatal errors
gettext: docs/locales/ja/LC_MESSAGES/story-class.po:527: end-of-line within string
docs/locales/ja/LC_MESSAGES/story-class.po:527:3: syntax error
docs/locales/ja/LC_MESSAGES/story-class.po:528: keyword "ctm" unknown
docs/locales/ja/LC_MESSAGES/story-class.po:530: end-of-line within string
docs/locales/ja/LC_MESSAGES/story-class.po:557: end-of-line within string
docs/locales/ja/LC_MESSAGES/story-class.po:557:3: syntax error
docs/locales/ja/LC_MESSAGES/story-class.po:558: end-of-line within string
msgfmt: found 7 fatal errors
gettext: docs/locales/ja/LC_MESSAGES/textwriter.po:296: keyword "en" unknown
docs/locales/ja/LC_MESSAGES/textwriter.po:296:43: syntax error
msgfmt: found 2 fatal errors
gettext: docs/locales/ja/LC_MESSAGES/widget.po:300: end-of-line within string
docs/locales/ja/LC_MESSAGES/widget.po:300:3: syntax error
docs/locales/ja/LC_MESSAGES/widget.po:301: end-of-line within string
docs/locales/ja/LC_MESSAGES/widget.po:332: end-of-line within string
docs/locales/ja/LC_MESSAGES/widget.po:333: end-of-line within string
msgfmt: found 5 fatal errors
gettext: docs/locales/ja/LC_MESSAGES/xml-class.po:333: keyword "sans" unknown
docs/locales/ja/LC_MESSAGES/xml-class.po:333:50: syntax error
docs/locales/ja/LC_MESSAGES/xml-class.po:418:57: syntax error
docs/locales/ja/LC_MESSAGES/xml-class.po:426:12: syntax error
docs/locales/ja/LC_MESSAGES/xml-class.po:731: keyword "red" unknown
docs/locales/ja/LC_MESSAGES/xml-class.po:731:37: syntax error
msgfmt: found 6 fatal errors
      

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.