“Gtk-WARNING **: Locale not supported by C library

Background information:

I use Mac OSX Yosemite.

I've installed gtk+ using brew install gtk+ and fixed the errors using export PKG_CONFIG_PATH=/opt/X11/lib/pkgconfig.

I've build my program using make but when I try to run it I get a warning like this:

(process:16182): Gtk-WARNING **: Locale not supported by C library.
        Using the fallback 'C' locale.

Problem: I cannot find any solutions for OS X for this particular problem.

What I've learnt so far:

In this thread (Gtk-WARNING **: Locale not supported by C library. while using several Python modules (mayavi, spectral)) they suggests using:
ipython --pylab=wx instead of ipython --pylab=osx
but I've got no idea how python can be related to my problem (my program is written in C - the same applies to gtk+ I guess)
You can find a lot of threads on this issue like this one: (Gtk-WARNING **: Locale not supported by C library. when starting apps from the commandline) but they mainly refer to Linux and/or Ubuntu.
Most of the answers use a command like this sooner or later:
```
 sudo dpkg-reconfigure locales
```
But there is no dpkg on OS X.

The issue here is that the environment variable LANG is not set, because I've mixed settings for Polish and English in the Language and Region section in System Preferences.

Quoting @KenThomases who helped me a lot with this problem:

There's not going to be any C library locale defined for the English language in Poland (i.e. en_PL.UTF-8). That's why Terminal is not setting LANG for you in your shells, even though you have "Set locale environment variables on startup" enabled.

You can read the in-depth solution here:
Is it bad that LANG and LC_ALL are empty when running `locale -a` on OS X Yosemite?