i18n: fix dead code in Accept-Language workaround from 7c7d6b5c07c7 AppConfig is just providing defaults, and the .ini file has not been read yet. There is thus no point in checking if i18n.lang has been set, and the code would thus *always* set i18n.lang=en ... but that was fine, as it just is a default.

# requirements.txt file for use as "pip install -r requirements.txt" as a
# readthedocs compatible alternative to "pip install -e ." which is a working
# alternative to "setup.py develop" which doesn't work with Mercurial 3.7
.