There is no Belarusian language in the game. It is available in Java Edition, but not in Bedrock. The modern youth of Belarus is very interested in the resumption of the use of the Belarusian language in everyday life, however, not all apps/games contribute to this, including Minecraft Bedrock.

Steps to reproduce (Bedrock)

1. Launch game
2. Go to settings 
3. Languages ​​tab
4. You won't see Belarusian in the list

Steps to reproduce (Java)

1. Launch game
2. Go to settings
3. Languages section
4. You will see Belarusian in the list