Language switcher for Craft CMS 3. More info: http://craftsnippets.com/articles/universal-language-switcher-for-craft-cms

language_switcher.twig

{# v2 #}
{# http://craftsnippets.com/articles/universal-language-switcher-for-craft-cms #}

{# logic #}
{% set currentElement = craft.app.urlManager.matchedElement %}
{% set sites = craft.app.getSites().getGroupById(currentSite.groupId).getSites() %}
{% set switcherLinks = [] %}

{% for site in sites if site.baseUrl is not empty %}
    
    {% set title = craft.app.i18n.getLocaleById(site.language).nativeName %}
    {% set url = site.getBaseUrl() %}
    {% if currentElement %}
        {% set otherLocaleElement = craft.app.getElements().getElementById(currentElement.id, currentElement.className(), site.id) %}
        {% if otherLocaleElement and otherLocaleElement.enabledForSite %}
            {% set url = otherLocaleElement.url %}
        {% endif %}
    {% endif %}

    {% set switcherLinks = switcherLinks|merge([{
        url: url, 
        title: title, 
        countryCode: site.language|split('-')|last,
        current: site.id == currentSite.id ? true : false,
        language: site.language,
    }]) %}

{% endfor %}

{# outputting html #}
{% if switcherLinks|length > 1 %}
<nav aria-label="{{'Switch language'|t}}" aria-role="navigation">
<ul>
{% for switcherLink in switcherLinks %}
    <li class="{{switcherLink.current ? 'is-active'}}">
        <a href="{{switcherLink.url}}" hreflang="{{switcherLink.language}}" lang="{{switcherLink.language}}">
        <span>{{ switcherLink.title }}</span>
        <span class="flag-icon flag-icon-{{switcherLink.countryCode}}"></span>
        </a>
    </li>
{% endfor %}
</ul>
</nav>
{% endif %}