Hi @Helikopter
If a word is underlined as being spelled wrong and you are sure it is right, you can right click and choose “add to dictionary”, which will create a custom dictionary entry.
To delete from the list of words you put in you can go to Settings | Editor | Spell Check | Gear Icon
From Obsidian Help doc:
“the gear next to Spell Check reveals the custom dictionary, so you can remove words you may have accidentally added.”
There doesn’t seem to be any way to bulk import words though. And I don’t know where the custom dictionary file is stored or it’s format.
This is afik the most that can be done concerning custom spellcheck.