The gtkmm text decode type is Unicode. Avoid using strlen().

Demo

Simple code

#include <gtkmm.h>

int main(int argc, char *argv[])
{
  auto app = Gtk::Application::create(argc, argv, "copyright");

  Gtk::Window window;
  Glib::ustring str("Test string");
  window.set_default_size(200, 200);

  Gtk::Button *btn = new Gtk::Button (str);
  btn->show();
  window.add(*btn);

  return app->run(window);
}