Since vim 7 has had it's own spellcheck. Finnish language was not supported, so Teemu Likonen wrote vimchant (available at vim.org), but it has not been maintained, and does not work anymore.
To get finnish spellcheck to work with vim 9,0 which Debian 12 has, install pacakage tmispell-voikko and get vimspell from https://www.vim.org/scripts/script.php?script_id=465
EDIT: also vimspell is not maintained, but works for now
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