Language switcher for Craft CMS 4. More info: http://craftsnippets.com/articles/universal-language-switcher-for-craft-cms

language-switcher.twig

{# v3 #}
{# http://craftsnippets.com/articles/universal-language-switcher-for-craft-cms #}

{# logic #}
{% set currentElement = craft.app.urlManager.matchedElement %}
{% set sites = craft.app.sites.getGroupById(currentSite.groupId).getSites() %}
{% set switcherLinks = [] %}
{% for site in sites|filter(x => x.baseUrl is not empty) %}
    {% set title = craft.app.i18n.getLocaleById(site.language).displayName %}
    {% set url = site.getBaseUrl() %}
    {% if currentElement %}
        {% set otherLocaleElement = craft.app.elements.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 %}
    {% tag 'nav' with {
        'aria-label': 'Switch language'|t,
        'aria-role': 'navigation',
    } %}
        {% tag 'ul' %}
            {% for switcherLink in switcherLinks %}
                {% tag 'li' with {
                    'class': switcherLink.current ? 'is-active',
                } %}
                    {% tag 'a' with {
                        'href': switcherLink.url,
                        'hreflang': switcherLink.language,
                        'lang': switcherLink.language,
                    } %}
                        {% tag 'span' %}
                            {{ switcherLink.title }}
                        {% endtag %}
                        {% tag 'span' with {
                            'class': 'flag-icon flag-icon-' ~ switcherLink.countryCode,
                        } %}
                        {% endtag %}
                    {% endtag %}
                {% endtag %}
            {% endfor %}
        {% endtag %}
    {% endtag %}
{% endif %}