Simredo has a spell checking function. The screen shot above shows misspelled words underlined in red. Currently, spelling dictionaries for English, Esperanto and Latin are included.
Since version 4.7, it is no longer necessary to install spelling dictionaries. The install-program includes these files (dictionary.zip, vortaro.zip, lexicon.zip), which it copies to the working folder 'simredo'.
To activate the spell checker, select 'Spell Check' under the 'Other' menu. There are four choices: Off, English, Esperanto and Latin.