From page 14 of the user manual:

1. Take the Roomba off its base and press Clean. The lights should turn on.
2. Press and hold Dock until the Roomba beeps and says the current language (about 3 seconds).
3. Press Clean until you hear the language you want.
4. Press and hold Clean to turn off the Roomba and confirm your language choice.

I have attached the full user manual for your vacuum below.

IRobot Roomba 800 Manual