First we create an object containing all translate texts

const translates = {
    fa: {
        blog: {
            header: {
                title: "وبلاگ محمدرضا خسرویان",
            },
        },
    },
    en: {
        blog: {
            header: {
                title: "Mohammad Reza Khosravian Blog",
            },
        },
    }
}

Then we create a function to get lang attribute of html documente element

const getHtmlLang = () => document.documentElement.lang

Finally we create a t (translate) function

function t(key, lang) {

    // get document language
    const language = lang || getHtmlLang() || "en";

    // take out locale translate texts
    const texts = translates[language];

    // find text
    return key.split('.').reduce((o, i) => {
        if (o) return o[i]
    }, texts)
}

Usage:

console.log( t("blog.header.title") ) // 'Mohammad Reza Khosravian Blog'

// Or

console.log( t("blog.header.title", "fa") ) // 'وبلاگ محمدرضا خسرویان'

// Or

console.log( t("blog.header.something", "en") ) // undefind