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 arduino-cli 's PO files:

source does not specify a priority
gettext: internal/locales/data/ar.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/be.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/de.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/en.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/es.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/fi.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/fr.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/he.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/it_IT.po: warning: Charset missing in header.
                                                                                            Message conversion to user's charset will not work.
gettext: internal/locales/data/ja.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/kk.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/ko.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/lb.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/mn.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/my_MM.po: warning: Charset missing in header.
                                                                                            Message conversion to user's charset will not work.
gettext: internal/locales/data/ne.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/pl.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/pt.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/pt_BR.po: warning: Charset missing in header.
                                                                                            Message conversion to user's charset will not work.
gettext: internal/locales/data/ru.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/si.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/zh.po: warning: Charset missing in header.
                                                                                         Message conversion to user's charset will not work.
gettext: internal/locales/data/zh_TW.po: warning: Charset missing in header.
                                                                                            Message conversion to user's charset will not work.
      

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.