Set language not working select2

Add script like this after select2 plugin

$.fn.select2.amd.define('select2/i18n/ru',[],function () {
    // Russian
    return {
        errorLoading: function () {
            return 'Результат не может быть загружен.';
        },
        inputTooLong: function (args) {
            var overChars = args.input.length - args.maximum;
            var message = 'Пожалуйста, удалите ' + overChars + ' символ';
            if (overChars >= 2 && overChars <= 4) {
                message += 'а';
            } else if (overChars >= 5) {
                message += 'ов';
            }
            return message;
        },
        inputTooShort: function (args) {
            var remainingChars = args.minimum - args.input.length;

            var message = 'Пожалуйста, введите ' + remainingChars + ' или более символов';

            return message;
        },
        loadingMore: function () {
            return 'Загружаем ещё ресурсы…';
        },
        maximumSelected: function (args) {
            var message = 'Вы можете выбрать ' + args.maximum + ' элемент';

            if (args.maximum  >= 2 && args.maximum <= 4) {
                message += 'а';
            } else if (args.maximum >= 5) {
                message += 'ов';
            }

            return message;
        },
        noResults: function () {
          return 'Ничего не найдено';
        },
        searching: function () {
          return 'Поиск…';
        }
    };
});

set option

$(".js-basic-multiple").select2({
    language: "ru"
});

If you use Webpack, you can set the default language like that (example for french) :

require('select2');
$.fn.select2.amd.define('select2/i18n/fr',[],require("select2/src/js/select2/i18n/fr"));

<script src="libs/select2-4.0.2-rc.1/dist/js/i18n/<here-goes-language>.js"></script>

The include above was missing as Keving Brown said. As the example quoted in the question the file for spanish would be "es.js"

CDN scripts:

<script src="https://cdn.jsdelivr.net/npm/[email protected]/dist/js/i18n/[culture-name].js"></script>