The Arc Studio desktop app now uses the languages from your operating system.
PC users can install new language packs. Please see this article for more: https://support.microsoft.com/en-us/windows/hardware/input-devices/manage-the-language-and-keyboard-input-layout-settings-in-windows
Spell-check is currently only available in the desktop version of the app. The web version will use your browser’s default spell-check instead.
See more:
How Do I Change the Spell Check Language?