One of Neovim's problems is the broken spell-file download.
This plugin adds a easy way to manually install the missing files.

1. Select your Language (example: :set spelllang=pl)
2. Run :DownloadSpellLang

Lazy.nvim:

{ 'jan9103/download-lang-files.nvim',
  lazy = true, cmd = 'DownloadSpellLang',
  dependencies = {'nvim-lua/plenary.nvim'},
  config = function() require('download-lang-files').setup() end,
}

Packer.nvim:

use { 'jan9103/download-lang-files.nvim',
  cmd = 'DownloadSpellLang',
  requires = {'nvim-lua/plenary.nvim'},
  config = function() require('download-lang-files').setup() end,
}

The ftp.vim.org http-mirror used for downloading.
The link has to end with a / and be the direct parent of the .spl files.
You can find a server list at: https://www.vim.org/mirrors.php
Default: https://ftp.pl.vim.org/pub/vim/runtime/spell/