Game:1.18.2

en_US.json

{
"key":"value"
}

new TranslatableText("key")

in the dev:value

in the game:key(get translate fail

en_us.json

{
"key":"value"
}

new TranslatableText("key")

in the dev:value

in the game:value

The names of language JSON files should be lowercase. The development environment is problematic here for accepting en_US.json as a valid alternative to en_us.json.