Hello Walter, Thanks for the reply!
Actually I'm not using GUIDE. Here is an example for the part I'm working on:
lang_eng = uicontrol('style', 'pushbutton',...
'fontsize', 12,...
'string', 'English',...
'units','normalized',...
'position', [0.55 0.5 0.2 0.1],...
'callback', 'lang = eng', 'callback', 'close all');
Obviously it doesn't work that way :)