In D22069#526284, @ngraham wrote:

I actually don't know how to properly install other language stuff. When I add French in the languages KCM, I suffer from https://bugs.kde.org/show_bug.cgi?id=327757.

What is the correct way to install the language?

I think this problem is just because you have installed a new version of ki18n (libKF5I18n.so.*). I get it too (also when compiling and installing versions of ki18n *before* the patch), so I guess the applications (or some libraries they depend on) have to be recompiled after updating ki18n.