brew install gtk+
(process:16182): Gtk-WARNING **: Locale not supported by C library.
Using the fallback 'C' locale.
sudo dpkg-reconfigure locales
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 Mac OSX Yosemite?