Spell checking in the Phrontex editor is handled by your browser, not by Phrontex. The details vary a little between browsers, but in general —

  1. Configure the browser through the browser Settings or by displaying the browser context menu. 
    • Set the language
    • Install the dictionary if necessary
    • Enable Check spelling.
  2. The browser will indicate what it thinks are mis-spellings with a squiggly underline.
  3. See the suggested corrections and the Add to dictionary options by pointing at the mis-spelling, then displaying the browser context menu.

To display the browser context menu

Use ctrl-right-click