Click on english word to translate it into russian. Translation will be logged into console.

trnslt.js

/**
 * <a href='http://api.yandex.ru/dictionary/'>Реализовано с помощью сервиса «API «Яндекс.Словарь»</a>
 */
(function() {
    const YANDEX_DICTIONARY_KEY = 'dict.1.1.20150130T172350Z.0895b15babe24c90.431f35fa9af476be1b6d14d80f3809ab693a5970';
    const DICTIONARY_URL = `https://dictionary.yandex.net/api/v1/dicservice.json/lookup?key=${YANDEX_DICTIONARY_KEY}&lang=en-ru`

    function translate(word) {
        fetch(`${DICTIONARY_URL}&text=${word}`)
            .then(res => res.json())
            .then(data => {
                if (!data.def.length &&
                    word[word.length-1] === 's' &&
                    word[word.length-2] !== 's')
                {
                    // yandex.dictionary is really bad with plurals, so cut "s" ending and try again
                    word = word.substr(0, word.length-1);
                    translate(word);
                    return;
                }

                const translation = data.def[0] &&
                    data.def[0].tr &&
                    data.def[0].tr[0] &&
                    data.def[0].tr[0].text;

                const translatedStyle = 'color: #c55';
                console.log('%s — %c%s', word, translatedStyle, translation);
            });
    }

    document.addEventListener('click', evt => {
        const selection = window.getSelection();
        selection.modify('move', 'backward', 'word');
        selection.modify('extend', 'forward', 'word');
        const word = selection.toString();
        translate(word);
    });
})();