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:

Install JSpell Evolution AJAX spell checker

The steps to install JSpell Evolution AJAX spell checker are detailed over here.

Ensure the JSpell service is running

You need to ensure that the JSpell service is running as detailed over here.

Grant Write access for the JSpell custom dictionary folder to Network User

It is necessary to grant the user "Network User" write access to the "lexicons" folder in the "JSpell Spell Checker" directory (usually "C:\Program Files (x86)\JSpell Spell Checker\lexicons"). Details about this are given here.

Check if JSpell and the custom dictionary works

Finally, you need to check if the JSpell spell checker as well as the custom dictionary are working as expected. The steps are detailed over here.

Switching the spell checker dictionary (if required)

You can opt to switch the spell checker later if required as detailed here.