I use Kile's automatic spell highlighting a lot. For some reason, it keeps complaining about ubiquitous verbiage like 'advective' or 'metachronal', which made me dig/google for the personal dictionary location.
According to https://forum.kde.org/viewtopic.php?f=20&t=98628, it's text files in the
~/.config/enchant folder, so for each localisation you're using, just edit the word list in e.g. 'en_GB.dic'.
For changes in the list to take effect, switch to a different localisation and back again.