If an offline spell checker functionality is needed, you can download JSpell Evolution AJAX Spell Checker from http://www.jspell.com/ajax-spell-checker.html.
It may be desirable to install an offline spell checker for one of the two reasons:
Finally, you need to check if the JSpell spell checker as well as the custom dictionary are working as expected. Here is how this can be done Check if JSpell and the custom dictionary works.
You can opt to switch the spell checker later if required as detailed here Switching the spell checker dictionary.