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 jupyter-notebook 's PO files:

gettext: notebook/i18n/fr_FR/LC_MESSAGES/nbui.po:692: duplicate message definition...
notebook/i18n/fr_FR/LC_MESSAGES/nbui.po:581: ...this is the location of the first definition
notebook/i18n/fr_FR/LC_MESSAGES/nbui.po:700: duplicate message definition...
notebook/i18n/fr_FR/LC_MESSAGES/nbui.po:577: ...this is the location of the first definition
msgfmt: found 2 fatal errors
gettext: notebook/i18n/ja_JP/LC_MESSAGES/nbui.po:688: duplicate message definition...
notebook/i18n/ja_JP/LC_MESSAGES/nbui.po:576: ...this is the location of the first definition
notebook/i18n/ja_JP/LC_MESSAGES/nbui.po:696: duplicate message definition...
notebook/i18n/ja_JP/LC_MESSAGES/nbui.po:572: ...this is the location of the first definition
msgfmt: found 2 fatal errors
gettext: notebook/i18n/nbui.pot:682: duplicate message definition...
notebook/i18n/nbui.pot:570: ...this is the location of the first definition
notebook/i18n/nbui.pot:690: duplicate message definition...
notebook/i18n/nbui.pot:566: ...this is the location of the first definition
msgfmt: found 2 fatal errors
gettext: notebook/i18n/nl/LC_MESSAGES/nbui.po:693: duplicate message definition...
notebook/i18n/nl/LC_MESSAGES/nbui.po:582: ...this is the location of the first definition
notebook/i18n/nl/LC_MESSAGES/nbui.po:701: duplicate message definition...
notebook/i18n/nl/LC_MESSAGES/nbui.po:578: ...this is the location of the first definition
msgfmt: found 2 fatal errors
gettext: notebook/i18n/zh_CN/LC_MESSAGES/nbui.po:683: duplicate message definition...
notebook/i18n/zh_CN/LC_MESSAGES/nbui.po:571: ...this is the location of the first definition
notebook/i18n/zh_CN/LC_MESSAGES/nbui.po:691: duplicate message definition...
notebook/i18n/zh_CN/LC_MESSAGES/nbui.po:567: ...this is the location of the first definition
msgfmt: found 2 fatal errors
gettext: notebook/i18n/zh_CN/LC_MESSAGES/notebook.po:156: 'msgid' and 'msgstr' entries do not both end with '\n'
notebook/i18n/zh_CN/LC_MESSAGES/notebook.po:163: 'msgid' and 'msgstr' entries do not both end with '\n'
notebook/i18n/zh_CN/LC_MESSAGES/notebook.po:186: 'msgid' and 'msgstr' entries do not both begin with '\n'
notebook/i18n/zh_CN/LC_MESSAGES/notebook.po:297: 'msgid' and 'msgstr' entries do not both end with '\n'
msgfmt: found 5 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.