language-selector.js

var element = document.querySelector('[data-module-language-selector]');

if (element) {
  element.addEventListener('change', (event) => {
    window.location = event.target.value;
  });
}

language-selector.liquid

<select data-module-language-selector>
  {% for locale in shop.published_locales %}
    {% assign locale_active = false %}

    {% if locale.iso_code == request.locale.iso_code %}
      {% assign locale_active = true %}
    {% endif %}

    <option value="{{ locale.root_url }}" {% if locale_active %}selected{% endif %}>
      {{ locale.name }}
    </option>
  {% endfor %}
</select>