Just create a virtual keyboard with all the foreign characters (non ASCII characters).
Then each key's click event put
textbox1.Text = textbox1.text + button1.Text;
This is C#, not WPF, so I'm not sure if the code is exactly the same.
support those strange characters.
P.S. You might want a few virtual keyboards, changeable by a drop down menu. e.g. Chinese, Arabic, Latin etc.