Notability's spellchecking uses the language of the keyboard that you're typing with.
You can add and change keyboards by following these steps:
- Go to "Settings > General > Keyboard > Keyboards > Add New Keyboard…".
- Tap the language of the keyboard you'd like to add.
- Open Notability and start typing.
- Press and hold the globe key near the bottom left corner of the keyboard.
- Tap the language you'd like to use from the menu that appears.