If you have more than one Debian ispell dictionary package installed, you should use update-ispell-dictionary to select the default one. This script is also run for you any time you install a new Debian ispell dictionary package.
This script makes extensive use of 'update-alternatives' from the dpkg suite of programs. Priority information for each of the alternatives is stored, read and acted upon by 'update-alternatives'. IMPORTANT: All ispell dictionary packages should install themselves with priority 10. This script will then assign priority 999 to the chosen default, and re-run update-alternatives.