指定したDOM配下の任意の文字列を置換する関数

translate.js

export default (rootElement, lang = 'ja') => {

    const texts = [
        {
            before: new RegExp('Parallel translation', 'g'),
            after: { ja: '対訳' }
        },
    ];

    const translate = node => {
        for (let i = 0; i < texts.length; i++) {
            if (node.nodeValue.match(texts[i].before)) {
                node.nodeValue = node.nodeValue.replace(texts[i].before, texts[i].after[lang]);
                break;
            }
        }
    }

    const dig = node => {
        if (node.nodeType === Node.ELEMENT_NODE) {
            for (let i = 0; i < node.childNodes.length; i++) {
                dig(node.childNodes[i]);
            }
        } else if (node.nodeType === Node.TEXT_NODE) {
            translate(node);
        }
    }

    const observer = new MutationObserver((targetNodes, observer) => {
        for (let i = 0; i < targetNodes.length; i++) {
            dig(targetNodes[i].target)
        }
    });
    observer.observe(rootElement, { childList: true, subtree: true });
}