jinja2.exceptions.UndefinedError: 'gettext' is undefined

I’m using Jinja2 2.7.3 inderectly through pelican and I get this error with templates using {% trans% } directive. So be sure it’s not related to pelican, i’ve created this small test file (https://gist.github.com/njouanin/36292368ad15f9b87ba0). When I run it with python interpreter (3.4.1) I get the following exception :

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/nico/Brassage/Beerfactory/env-site/lib/python3.4/site-packages/jinja2/environment.py", line 969, in render
    return self.environment.handle_exception(exc_info, True)
  File "/Users/nico/Brassage/Beerfactory/env-site/lib/python3.4/site-packages/jinja2/environment.py",     line 742, in handle_exception
    reraise(exc_type, exc_value, tb)
  File "/Users/nico/Brassage/Beerfactory/env-site/lib/python3.4/site-packages/jinja2/_compat.py", line 36, in reraise
    raise value.with_traceback(tb)
  File "theme/templates/base.html", line 50, in <module>
    {% block header_left_section %}
  File "theme/templates/base.html", line 60, in <module>
    <h2 id="site-description">{{ gettext('Brasser libre !') }}</h2><!-- #site-description -->
jinja2.exceptions.UndefinedError: 'gettext' is undefined

I’ve googled this issue which appear to be related to gettext library which is missing. Here importing gettext module in python works fine, so I guess it is installed. Here is my environnement : MacOs 10.9 Python 3.4.1 Jinja2 : 2.7.3

[mitsuhiko]

I’m going to close this issue. This happens if the i18n extension is installed but no gettext function is provided. This is a configuration problem of whatever integrated Jinja into a project.

[davidism]

You (or the library that’s configuring Jinja for you) need to install the gettext functions after enabling the i18n extension. This is shown in the docs https://jinja.palletsprojects.com/en/3.0.x/extensions/#i18n-extension

translations = get_gettext_translations()
env = Environment(extensions=["jinja2.ext.i18n"])
env.install_gettext_translations(translations, newstyle=True)