Spell check is not working for me (openSUSE Tumbleweed official repository). The dictionaries /usr/share/qt5/qtwebengine_dictionaries/*.bdic are found by Falkon and listed in preferences.
You must select language you want to use (just click on it)! You can chose several languages alongside.
(In reply to private2.6.3 from comment #1) > You must select language you want to use (just click on it)! Thank you, it works. Maybe the preference dialog should have a hint that I have to select from the listed languages.