This should really be on the app interface!
Please everyone, upvote Introduce UI elements to select spellcheckers and Custom spellcheck dictionary and spellcheck languages
I can confirm this is correct for Linux, but it would be nice to compile answers to other platforms (confirm or enhance and I’ll edit this):
On Windows C:\users\<your-username>\AppData\Roaming\Logseq\Preferences
On Linux ~/.config/Logseq/Preferences (notice it is not the .logseq folder!)
On MacOS ? Logseq/Preferences