Extend Programmer's Guide with notes about influence of setlocale()

Summary:
Collect knowlegde found when investigating why i18n calls before line
QApplication app;
do not return translated strings.

Perhaps needs more prominent place to warn about the issues.
But having it documented somewhere will be a start.

Reviewers: Frameworks, ilic, aacid

Reviewed By: ilic, aacid

Subscribers: aacid

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D5455