To disable or enable spell checking, click the
button and run the Settings command from the main menu. In the Settings pane, in the Spelling and grammar block, clear or select the Check spelling checkbox.
When working in macOS, in the Tools > Spelling and grammar command menu, clear or select the Check spelling checkbox.

Check spelling basics
–The spell check dictionary is selected automatically by the language of the first word in the paragraph.
–The red wavy line is used to underline:
•Words absent in the selected dictionary.
•Words with spelling mistakes.
–The spell checker runs every time you make changes to the text of the presentation.