en.json

{
    "hello_world": "Hello World",
    "hi": "Hi {{name}}!"
}

index.js

const newI18n = require('new-i18n');
const i18n = newI18n(__dirname, ['en', 'pt']); // You can use `.` instead of `__dirname`

console.log('English:', i18n('en', 'hello_world'));
console.log('Portuguese:', i18n('pt', 'hello_world'));

console.log(i18n('en', 'hi', { name: '...' }));
console.log(i18n('pt', 'hi', { name: '...' }));

pt.json

{
    "hello_world": "Olá Mundo",
    "hi": "Oi {{name}}!"
}