Can I change the language of the spell check?
In the Missive app for Mac, spell check language should be detected automatically with no need to customize it.
If the spell check appears to not be working, you may need to change your macOS Keyboard > Text Input preferences.
On Windows, you can customize spell check dictionaries to use up to 3 at the same time. To do so, write some random text in the draft composer or the chatbox, then right-click the text, and you will see the dictionary list you can customize. See the following image: