You cannot select an arbitrary name. :help 'spellfile' says:
It must end in ". {Encoding} .add".
So use this syntax (and make sure the spell directory exists):
:set spellfile=~/.vim/spell/en.utf-8.add
To encode, check the box :set encoding? (Technically, this is a buffer-local parameter of 'fileencoding' , but most of the default settings are for the first).
Ingo karkat
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