I created a new file called "zh_cn.lang" and put it in lang folder.
As follows

customloadingscreen.modstate.minecraft_init=Minecraft初始化中
customloadingscreen.modstate.construction=构建
customloadingscreen.modstate.pre_initialization=预初始化
customloadingscreen.modstate.initialization=初始化
customloadingscreen.modstate.post_initialization=后初始化
customloadingscreen.modstate.completed=完成
customloadingscreen.modstate.reloading_resource_packs=重新加载资源包
customloadingscreen.mcstate.modelloader.items=加载物品模型
customloadingscreen.mcstate.modelloader.blocks=加载方块模型
customloadingscreen.mcstate.modelloader.bake=烘焙模型
customloadingscreen.mcstate.texture.load=加载纹理
customloadingscreen.mcstate.texture.stitch=拼接纹理
customloadingscreen.loading=加载中
customloadingscreen.finishing=完成

The encoding used is UTF8.
But in the game, it's the color blocks.

Mod version is 1.12.2-1.4.1
English is translated by Google and may not be accurate.

Does 1.5.0-pre.1 work correctly?

Does 1.5.0-pre.1 work correctly?

yes,it's working.