If you're looking for the Fabric version of this mod, use the link below:

https://www.curseforge.com/minecraft/mc-mods/mclangsplit-fabric

This mod injects a configurable second language into all text strings in the game. This could be useful for learning a second language without having to keep switching between languages. This mod has been designed to be purely client-side.

There is a hotkey for cycling though the configured language modes (default R):

Original -> Alternate -> Both

Version 2.x changes (Minecraft 1.19.3 onwards)

As of v2.x of this mod, configuration will now be done in JSON instead of TOML.

{
  "language": "ja_jp",
  "ignoreKeys": ["menu.*"],
  "includeKeys": ["menu.singleplayer"],
  "ignoreTooltips": false,
  "mode": "BOTH"
}

Included alongside this change is support for Cloth Config.

Check out the Wiki for more information:
https://github.com/AntMCDev/MCLangSplit/wiki/Configuration

Author's Note:

This project is licensed under the MIT license - meaning that you can freely make changes and distribute the source code etc, however if you do so please link back to this project. Also, check out my skyblock mod if you're interested:

https://www.curseforge.com/minecraft/mc-mods/mcskyblock