i18n doesn't work at production environment on heroku

Question

I have seen more than one hundred posts about i18n issues and no solution seems to solve my problem.

I have an app running with Django 1.3.1 and it works Fine at my

Answer 1

I don't answer related to Heroku, but just to have most possibilities collected:

I have Debian 10 development and Debian 10 virtual server in production. Everything was ok in development, i18n failed in production.

Reason: I needed apt install gettext.

And in addition, be sure that .mo files are in git and not disabled by .gitignore (this answer is here already),