Run following commands
cd app/
mkdir translations
pybabel extract -F babel.cfg -k lazy_gettext -o translations/messages.pot .
pybabel init -i translations/messages.pot -d translations -l en
pybabel init -i translations/messages.pot -d translations -l es
pybabel init -i translations/messages.pot -d translations -l zh
pybabel compile -d translations
cd app/
pybabel update -i translations/messages.pot -d translations