I use this page's Basic Sample.
https://wiki.gnome.org/Projects/Vala/GTKSample

using Gtk;

int main (string[] args) {
    Gtk.init (ref args);

    var window = new Window ();
    window.title = "First GTK+ Program";
    window.border_width = 10;
    window.window_position = WindowPosition.CENTER;
    window.set_default_size (350, 70);
    window.destroy.connect (Gtk.main_quit);

    var button = new Button.with_label ("Click me!");
    button.clicked.connect (() => {
        button.label = "Thank you";
    });

    window.add (button);
    window.show_all ();

    Gtk.main ();
    return 0;
}

when I change 'window' to '窗口' or 'tm_窗口', the code will compilation failed. It says "syntax error, unexpected character". I checked the code. this error happened at line 1207.
------------------code begin--------------------
default:
	unichar u = ((string) current).get_char_validated ((long) (end - current));
	if (u != (unichar) (-1)) {
	current += u.to_utf8 (null);
	Report.error (get_source_reference (0), "syntax error, unexpected character");
} else {
	current++;
	Report.error (get_source_reference (0), "invalid UTF-8 character");
}
column++;
return read_token (out token_begin, out token_end);
------------------code end------------------------
It seems that vala support UTF-8 character while I got error here.