Spell-checking requires an extra (optional) perl module installed before it will work, but no other changes to the site, beyond configuration in the admin interface. The only language currently supported is English.
You will need the aspell program and the Text::Aspell perl module, which provides a perl interface to aspell. Aspell, its dictionary, and Text::Aspell should be installed according to the instructions in the install section (2.1.3).
Once the aspell program and perl module are installed, Apache must be stopped then started so it will find the spellchecking module in its initialization. No further Apache configuration is necessary.
Spellchecking is configured through the Site Controls Admin Tool (A.6), in the Spellchecker category. It is turned on using the variable spellcheck_enabled.
Whether spellchecking is on by default or must be explicitly turned on by the user at comment posting time is set using the variable spellcheck_default. The specific English variant used by default is set in the variable spellcheck_spelling. Both of these can be overridden by a user preference.
Permission to use the spellchecker is given using the perm use_spellcheck, in the Groups Admin Tool (A.12).