Yes, it spellchecks UK English when asked.
$ls -lha ~/.config/Deepdwn/settings.json
-rw-rw-r-- 1 ian ian 521 Dec 25 13:01 /home/ian/.config/Deepdwn/settings.json
If I look inside, I can see
"spellcheck_languages": [
"en-GB"
],
If I close the program and reopen, that's still there, but Tools/Preferences/Spellcheck languages only shows US English (and all my nice UK English documents show spelling errors!) As I say, other settings seem to be remembered successfully.