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

I was recently writing a mono application for OSX in MonoDevelop, using Gtk# as the GUI framework. The app seemed to work fine, but it would throw the following error each time it started:

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

Since I don’t like errors, even if I don’t understand their immediate effect, I did a bit of searching. My search led me to this StackOverflow post, whose answers suggested defining the following environment variables in the terminal:

export LC_ALL=en_US.UTF-8
export LANG=en_US.UTF-8

Indeed, setting those variables in a bash script, prior to the execution of my application, resolved the issue 🙂