How to switch languages with the i18next plugin?

Question

I am using Backbone.js in my application and the i18next plugin for my language switch function on my application. When I pass a value to the lng option in the init function call, then it translates my page correctly.

Now I want to do this dynamically via a language selector. I have a <select> of four languages and I want to pass the value of the selected language to the lng option of the init function.

Here is my code:

HTML

<div class="col-xs-6>
    <select class="form-control language-selector">
        <option value="de">Deutsch</option>
        <option value="en">English</option>
        <option value="fr">Français</option>
        <option value="it">Italiano</option>
    </select>
</div>

JavaScript

i18next.init({
        debug: true,
        languages: ['de','en','fr','it'],
        lng: 'de',  
        fallbackLng: false,
        load: 'current',
        resources: resBundle
    }, function(err, t){

});

'change .language-selector': function(e){
    e.preventDefault();
    i18next.setLng($(e.target).val(), (err, t) => {
        console.log(arguments);
        this.render();
    });
}

Answer 1

The function to change the language is i18next.changeLanguage. You only need to call it, there's no need to call init again or to "change the init options" as the options are attributes inside i18next and they are managed through the API (the functions).

i18next.init({
    lng: 'en',
    fallbackLng: ['en', 'de', 'fr', 'it'],
});

// catch the event and make changes accordingly
i18next.on('languageChanged', function(lng) {
    // E.g. set the moment locale with the current language
    moment.locale(lng);

    // then re-render your app
    app.render();
});

In the view with the language selector:

var LangView = Backbone.View.extend({
    events: {
        'change .language-selector': 'onLangChange',
    },

    onLangChange: function(e) {
        // only change the language
        i18next.changeLanguage(e.currentTarget.value);
    }
});

Proof of concept

var app = {};

app.translations = {
    "fr": {
        "translation": {
            "label": "Choisir une langue",
            "fr": "Français",
            "en": "Anglais"
        }
    },
    "en": {
        "translation": {
            "label": "Choose a language",
            "fr": "French",
            "en": "English"
        }
    }
};

i18next.init({
    lng: 'en',
    fallbackLng: ['en', 'fr'],
    resources: app.translations,
});

// catch the event and make changes accordingly
i18next.on('languageChanged', function(lng) {

    // then re-render your app
    app.view.render();
});

var LangView = Backbone.View.extend({
    template: _.template($('#selector').html()),
    langTemplate: _.template('<option value="<%= value %>"><%= name %></option>'),
    events: {
        'change .language-selector': 'onLangChange',
    },

    render: function() {
        this.$el.html(this.template({
            label: i18next.t('label')
        }));
      
        // cache the jQuery object of the select
        this.$selector = this.$('.language-selector');
      
        // then dynamically populate it
        this.populateSelector();

        return this;
    },

    populateSelector: function() {
        // for each languages in i18next, add an option to the select
        _.each(i18next.languages, this.addLanguage, this);
    },

    addLanguage: function(lang) {
        // adding the option with the translated names
        this.$selector.append(this.langTemplate({
            value: lang,
            name: i18next.t(lang),
        }));
    },

    onLangChange: function(e) {
        // change the language
        i18next.changeLanguage(e.currentTarget.value);
    }
});

app.view = new LangView();

$('#app').html(app.view.render().el);

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/underscore.js/1.8.3/underscore-min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/backbone.js/1.3.3/backbone-min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/i18next/4.1.1/i18next.min.js"></script>

    <div id="app"></div>
    <script type="text/template" id="selector">
        <label>
            <%=label %>
          </label>
        <select class="form-control language-selector"></select>
    </script>

Regarding translating the language names, take a look at Language of language names in the language selector?