Fix "Error: locale::facet::_S_create_c_locale name not valid" with en_US.UTF-8 missing

--- a/main.cpp
+++ b/main.cpp
@@ -134,7 +134,7 @@ int main(int argc, char *argv[])
 #if defined (Q_OS_WIN)
       std::locale::global (std::locale ("C"));
 #else
-      std::locale::global (std::locale ("en_US.UTF-8"));
+      std::locale::global (std::locale ("C.UTF-8"));
 #endif
 
       // Override programs executable basename as application name.
