Right-click a style in the Styles task pane.
Select Modify... from the context menu.
In the Modify Style dialog, click Format > Language...
Tick the check box "Do not check spelling or grammar".
Click OK twice.
Comments use the Comment Text style, so you can use the above method to disable spell checking in comments.
Remark: if you don't see the style you need in the Styles task pane, click Options... at the bottom of the task pane and select 'All styles' from the 'Select styles to show' dropdown, then click OK.
Regards, Hans Vogelaar