Based on D5209, which adds this feature to plasma-framework.

https://phabricator.kde.org/D5209

Assuming /usr/share/locale/fr/LC_MESSAGES/plasma_applet_org.kde.plasma.digitalclock.mo exists.

• LANGUAGE=fr plasmoidviewer -a org.kde.plasma.digitalclock
  • i18n() strings are translated into french.
  • Qt datetime formatting (Qt.formatDate(tzDate,"dddd")) like in digitalclock's tooltip uses the system locale.
• LANG=fr_CA.UTF-8 LANGUAGE=fr plasmoidviewer -a org.kde.plasma.digitalclock
  • i18n() strings are translated into french.
  • Qt datetime formatting uses the french canadian locale.

Before patch:

• LANGUAGE=fr plasmoidviewer -a ./package
  • i18n() strings are not translated.
• plasmoidviewer -a ./bundle.plasmoid
  • Does not work. Seems it needs to be extracted first for plasmoidviewer to work.

After patch:
First setup a test env that doesn't have translations elsewhere.

cp -r /usr/share/plasma/plasmoids/org.kde.plasma.volume .
cd org.kde.plasma.volume
mkdir -p contents/locale/fr/LC_MESSAGES
rm metadata.json
kwriteconfig5 --file "$(realpath metadata.desktop)" --group "Desktop Entry" --key "X-KDE-PluginInfo-Name" "org.kde.plasma.volumetest"
cd contents/locale/fr/LC_MESSAGES
cp /usr/share/locale/fr/LC_MESSAGES/plasma_applet_org.kde.plasma.volume.mo .
mv plasma_applet_org.kde.plasma.volume.mo plasma_applet_org.kde.plasma.volumetest.mo
cd ../../../..

• LANGUAGE=fr plasmoidviewer -a .
  • i18n() strings are translated into french, loaded from ./package/contents/locale/fr/LC_MESSAGES/plasma_applet_na.me.space.mo
• Tested when contents/locale doesn't exist.
• Tested when metadata.desktop and metadata.json doesn't exist.