The spell checker included with Help+Manual supports both manual spell checking (check topics or the entire project in a single session) and "live" spell checking, which highlights incorrectly spelled words as you type.
See Spell checking in the Creating and Editing Topics chapter for instructions on using both manual and live spell checking functions.
The spell checker uses Open Office dictionaries for spell checking and hyphenation. These dictionaries are free and are available for a very large number of languages.
Spell checking is supported almost everywhere in Help+Manual where you can enter text. Just right-click to display the context menu or click on the upper half of the Spelling tool in the Project tab to access.