Android
Turn On/Off Spell Checker auf Android
The spell checker is a separate function from autocorrect. Autocorrect changes words as you type; the spell checker only underlines words it does not recognize, so you can fix them yourself.
On most Android phones this option lives inside the keyboard app settings, not in the general language settings. If you use Gboard, follow this path:
Settings ➔ System ➔ Keyboard ➔ On-screen keyboard ➔ Gboard ➔ Text correction ➔ toggle Spell check on or off.
When to leave it off
If you type a mix of languages or a lot of slang, the spell checker can underline correct words constantly. Turning it off removes the distraction but also removes the warning for genuine typos.
If you use a third-party keyboard instead of Gboard, look for the same option under that app's own settings menu, usually labeled Text correction or Spelling.