Extract messages in a PO template file:
pybabel extract -F inginious/frontend/babel.cfg ./ -o inginious/frontend/i18n/messages.pot
Then update the PO files for each language:
pybabel update -i messages.pot -o inginious/frontend/i18n/en/LC_MESSAGES/messages.po -l en pybabel update -i messages.pot -o inginious/frontend/i18n/fr/LC_MESSAGES/messages.po -l fr ...
After modifying the .po file (example for the french translation):
pybabel compile -i inginious/frontend/i18n/fr/LC_MESSAGES/messages.po -o inginious/frontend/i18n/fr/LC_MESSAGES/messages.mo -l fr