The problem seems to be worst with the French AZERTY layout, where the digit keys procude letters if unshifted?  With the Swedish/Finnish layout, I can define groups all right; only the unselect keys (` and ^) are missing.

Some potential fixes or workarounds:

• When SDL reports a keypress where the keysym or the Unicode character is a digit, save the scancode from that event.  If later events occur with the same scancode, treat those as that digit too, even if the Unicode character now is different because of a changed shift state.
• Examine the keymap.  SDL 1.2 doesn't seem to have functions for that, so it would have to be implemented in platform-specific code.
• Add a keyboard layout drop-down list in the settings menu, near the language selector.
• Define default GroupKeys strings in the PO files.  This is somewhat inadvisable because people may want to play in a non-native language (because of bad translations) but with a native keyboard.

The numeric keys at the top of the keyboard work only for US english keyboards. For other keyboards, you have to set GroupKeys to the appropriate values in preferences.lua.

It's an annoying behaviour and I welcome improvements.