The Mi Box FAQ says:
WHAT LANGUAGE DOES VOICE REMOTE CONTROL SUPPORT?
Currently, it supports English, Chinese, Japanese, Korean.
Is this a Mi box or Android TV limitation?
If Mi box, why is this Mi box specific and not generic for Android TV?
If Android TV, why only these languages if Android on the phone supports many others?
Here are the speech and display languages as of version 6.0.1. I will update it if there are any additions in future updates.
The language list is typically the same as the ones on the Nexus 6P and 5X, but with Punjabi and Romansh as additional languages.