When you change to UK english, does the spellcheck appear to be correct (using UK spellings)?
If you make another settings change right now (like toggling autosave on or off), exit, and reopen Deepdwn, does that setting change “stick”, or has it reset back to the original value?
Is your settings file writeable? (~/.config/Deepdwn/settings.json)
When you make a change to your spellcheck language settings, is the settings file updated? spellcheck languages are stored under the spellcheck_languages key.