docs: outline the challenges of specifying a locale for services (Issue #340)

Trying to embed the most essential parts of
http://blog.dscpl.com.au/2014/09/setting-lang-and-lcall-when-using.html and
https://modwsgi.readthedocs.io/en/develop/configuration-directives/WSGIDaemonProcess.html
.

(some rewording by Thomas De Schampheleire)