Describe the bug

When using custom template overrides as described in https://www.sphinx-doc.org/en/master/development/templating.html

If the template contains translated strings, for instance:

{% extends "!layout.html" %}
{% block document %}
    {{ _('coucou') }}
    {{ super() }}
{% endblock %}

The strings are correctly extracted with the gettext builder (make gettext), but when the base language is not English, and the translated language is English, the translation catalog is not used. It works as expected when translating in another language than English (for example from French to German).

How to Reproduce

I made a reproducer in this git repo: https://github.com/n-peugnet/sphinx-template-en-translation-bug or this zip:
sphinx-template-en-translation-bug-main.zip

git clone https://github.com/n-peugnet/sphinx-template-en-translation-bug
cd sphinx-template-en-translation-bug
make html O=-Dlanguage=en

Then check the content of _build/html/index.html

Environment Information

Platform:              linux; (Linux-6.6.15-amd64-x86_64-with-glibc2.37)
Python version:        3.11.8 (main, Feb  7 2024, 21:52:08) [GCC 13.2.0])
Python implementation: CPython
Sphinx version:        7.3.0
Docutils version:      0.20.1
Jinja2 version:        3.1.3
Pygments version:      2.17.2

Sphinx is latest git version: v7.2.6-318-gd5baa46d8

Sphinx extensions

No response

Additional context

Notice that running make html O=-Dlanguage=de produces the expected result.