Using i18n in JavaScript

file.html

<div id="result">...</div>

file.js

/* _i18n.js */

appConfig = {
    locale: {
        code: 'enUS' // must reflect the property name of each locale file
    }
}

var getPropertyValue = function(obj, desc) {
    var arr = desc.split('.');
    while (arr.length && (obj = obj[arr.shift()]));
    return obj;
}

var text = function(name, params = {}) {
    var i18n = appConfig.locale[appConfig.locale.code](params);
    return getPropertyValue(i18n, name);
}

/* pt-br.js */

appConfig.locale.ptBR = function(params) {
    return {
        "about": "Sobre",
        "account": {
            "welcome": "Boas vindas "+params.username,
            "info": "Algo aqui."
        }
    };
}

/* en-us.js */

appConfig.locale.enUS = function(params) {
    return {
        "about": "About",
        "account": {
            "welcome": "Welcome "+params.username,
            "info": "Something here."
        }
    };
}

/* index.js */

var result = text('account.welcome', {username: 'Brynner'});
document.querySelector('#result').innerHTML = result;

https://jsfiddle.net/brynner/uhv0Lmyk/