GTK: default to the en_US locale if LANGUAGE is set and empty.

CONTRIBUTING.md

@@ -238,3 +238,13 @@ any other information that is requested:
 
 This will generate a large file called `core` in the current
 directory; it can be later re-loaded using `lldb -c core`.
+
+### Debugging GUI-related bugs on Linux
+
+There are several environment variables available that make crashes
+earlier and errors more informative. Before running SolveSpace, run
+the following commands in your shell:
+
+    export G_DEBUG=fatal_warnings
+    export LIBGL_DEBUG=1
+    export MESA_DEBUG=1

src/platform/gtkmain.cpp

@@ -1433,6 +1433,9 @@ int main(int argc, char** argv) {
     while(*langNames) {
         if(SetLocale(*langNames++)) break;
     }
+    if(!*langNames) {
+        SetLocale("en_US");
+    }
 
 #if defined(HAVE_SPACEWARE) && defined(GDK_WINDOWING_X11)
     if(GDK_IS_X11_DISPLAY(Gdk::Display::get_default()->gobj())) {