Diagnosis
The language was changed accidentally by holding the Volume Up and Play/Pause buttons during startup, or it was paired with a device that changed the setting via the Bose Connect app.
How to Fix It
Reset the language to English manually. Turn the speaker off. Press and hold the Volume Up button and the Play/Pause button simultaneously. While holding both, press the Power button once. Keep holding Volume Up and Play/Pause until you hear a tone. Release. The language is now reset to English.
If That Didn't Work
Use the Bose Connect app if you have it installed. Connect to the speaker within the app. Go to the speaker settings. You should find a language selection menu there.
Heads Up
The language reset procedure also clears your custom name for the speaker. You will need to rename it in your phone's Bluetooth settings after this.