[STUMP] window-send-string doesn't work with all apps

Hi,

Recently I replaced my usual terminal, Konsole, with Terminator and 
immediately noticed that my Stumpwm hotkeys which are supposed to insert 
various pieces of text to the current window, don't work anymore.

Here's a trivial example:
(defcommand insert-date () ()
     (window-send-string (multiple-value-bind
                           (s m h date month year)
                           (get-decoded-time)
                           (format nil "~d-~2,'0d-~2,'0d" year month 
date))))

This function inserts current date to Konsole, Firefox, Thunderbird, 
LibreOffice and many other applications, but doesn't work with 
Terminator, Xterm or Gnome-terminal. I tried to play with GTK_IM_MODULE, 
but it didn't help either.

Is there a way to make window-send-string to work with these stubborn 
terminals?