How do you get the spell check to work in falkon?

Go to Falkon's preferences (use the menu to the right of the address bar. One of the last options is about the spellchecker, you can enable it there. Notice that the spellchecker in Falkon is quite limited due to its dependence in QtWebEngine: it uses non standard dictionaries, just a few languages are supported and if you have more than one dictionary installed, it will check for all languages simultaneously.

EDIT: For more information on how to install dictionaries to use in Falkon, check this article.

Another Falkon dysfunction, at least in BSD installation the hard-wired dictionary directories don't exist.
To be functional, I suggest Falkon should allow the user to specify his dictionary directory/directories.