Done it:
juliomarin@Julio-Studio:~/Proyectos/QET/docs/source$ pip install --user sphinx-intl
/usr/local/lib/python2.7/dist-packages/pip/_vendor/requests/__init__.py:83: RequestsDependencyWarning: Old version of cryptography ([1, 2, 3]) may cause slowdown.
warnings.warn(warning, RequestsDependencyWarning)
DEPRECATION: Python 2.7 will reach the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 won't be maintained after that date. A future version of pip will drop support for Python 2.7.
Collecting sphinx-intl
Using cached https://files.pythonhosted.org/packages/a0/4c/fab5e1eeac6c0b534db888a8ce6f60fc0a88e798170f0448b0c7db7da16d/sphinx_intl-1.0.0-py2.py3-none-any.whl
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Installing collected packages: click, alabaster, MarkupSafe, Jinja2, snowballstemmer, pyparsing, packaging, pytz, babel, imagesize, Pygments, typing, docutils, urllib3, certifi, chardet, idna, requests, sphinxcontrib-websupport, sphinx, sphinx-intl
WARNING: The script pybabel is installed in '/home/juliomarin/.local/bin' which is not on PATH.
Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
WARNING: The script pygmentize is installed in '/home/juliomarin/.local/bin' which is not on PATH.
Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
WARNING: The script chardetect is installed in '/home/juliomarin/.local/bin' which is not on PATH.
Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
WARNING: The scripts sphinx-apidoc, sphinx-autogen, sphinx-build and sphinx-quickstart are installed in '/home/juliomarin/.local/bin' which is not on PATH.
Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
WARNING: The script sphinx-intl is installed in '/home/juliomarin/.local/bin' which is not on PATH.
Consider adding this directory to PATH or, if you prefer to suppress this warning, use --no-warn-script-location.
Successfully installed Jinja2-2.10.1 MarkupSafe-1.1.1 Pygments-2.4.2 alabaster-0.7.12 babel-2.7.0 certifi-2019.6.16 chardet-3.0.4 click-7.0 docutils-0.14 idna-2.8 imagesize-1.1.0 packaging-19.0 pyparsing-2.4.0 pytz-2019.1 requests-2.22.0 snowballstemmer-1.2.1 sphinx-1.8.5 sphinx-intl-1.0.0 sphinxcontrib-websupport-1.1.2 typing-3.7.4 urllib3-1.25.3
I think now it is only needed to add it to PATH, isn't?