Mailspring automatically detects the language you're writing in and spellchecks in that language. On Linux and older versions of Windows where dictionaries are not always available, Mailspring downloads Hunspell dictionaries on-the-fly and it may take a few minutes for spellcheck to start working. If Mailspring doesn't detect the language that you're typing in, please write a draft in the language of your choice and allow Mailspring to try to download the language in the background.
On Mac OS X and Windows 10, Mailspring will default to your operating system language for spell checking your drafts. To change your spellchecking language, you'll need to change your system language.
On Linux and older versions of Windows, Mailspring will spell check in English when your system language is English. System dictionaries are not available for other languages, and Mailspring will not perform spell check if your system language is not English. We're working to expand spell check support for other languages on Linux and Windows, and you can follow the discussion here: https://github.com/nylas/N1/issues/1392