Falkon is a cross-platform Qt web browser previously known as QupZilla. It uses QtWebEngine rendering engine.
Spell checking in Falkon currently supports only basic functionality and also needs a custom format of dictionaries. This is a limitation that comes from used web engine.
Dictionaries needs to be in special format that uses Chromium, with suffix .bdic. QtWebEngine comes with utility to convert Hunspell dictionaries (.dic) to this format.
qwebengine_convert_dict /path/to/hunspell/dict.dic /output/path/to/created.bdic
Created bdic dictionaries then need to be moved to one of the directories shown in the Preferences.
Currently, as of QtWebEngine 5.8, following useful features are missing: