Polymail’s Web and Desktop apps provide spell check based on the languages installed on your device. (Please note, Polymail does not offer grammar checking.)
On macOS, you can see which languages are installed by checking your computer’s Language and Region settings. You’ll have a default language set, along with any additional languages you’ve added.
You can write emails using multiple languages, but keep in mind that spell check may not accurately detect or correct different languages used within the same sentence.