Diagnosis
The robot's voice language is tied to the smartphone language setting in the iRobot app, not a separate setting on the robot. If you (or someone else) changed your phone's system language, the robot followed suit.
How to Fix It
Open the iRobot HOME app. Tap the three-line menu in the top left, then go to your robot's settings. Look for "Voice & Sound" or a similar option. The language setting here will be grayed out. To change it, you must first change your smartphone's system language. Go to your phone's Settings > General > Language & Region, and set it to your preferred language. Then, force-close and reopen the iRobot app. The voice option should now be adjustable.
If That Didn't Work
As a nuclear option, you can reset the robot's Wi-Fi and re-add it to the app. Press and hold the HOME and SPOT CLEAN buttons on the robot for 10 seconds until it plays a tone. This resets network settings but preserves maps. Set up the robot again in the app with your phone set to the correct language.
Heads Up
If multiple people in your household use the app on their phones with different language settings, the robot's voice will change based on which phone last sent it a command. There's no way to lock it to one language independently.