To change the language, modify the flex file "scanner.l" and the bison file "parser.y".
Then, regenerate the two files "lex.yy.c" and "parser.tab.c" by :
_ Opening a terminal and typing 
                 cd path_to_this_Qt_project                 (e.g. cd /home/user/Desktop/ibex-lib1.19_qt_sivia_flex_bison_windows_linux/sivia_flex_bison_ibex)
                 flex scanner.l
                 bison parser.y
for Linux. Note that you may need to install the flex and bison packages (e.g. for Ubuntu 10.1 : via Synaptic, or the command sudo apt-get install flex bison...) to succeed.
_ Double-clicking on flexbison_win32.bat for Windows (the flex and bison executables are provided in the flexbison_win32 directory). You can also open a command prompt and type : 
                 cd /d "path_to_this_Qt_project"                 (e.g. cd /d "C:\Users\Administrator\Desktop\ibex-lib1.19_qt_sivia_flex_bison_windows_linux\sivia_flex_bison_ibex)
                 flexbison_win32\flex scanner.l
                 flexbison_win32\bison parser.y

Note : The Qt SDK version for this project was qt-sdk-linux-x86-opensource-2010.05.1.bin for Linux (Ubuntu 10.1 32 bit) and qt-sdk-win-opensource-2010.05.exe for Windows (7 64 bit or XP 32 bit).
