flink-python/docs/_templates/lang-switcher.html

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{# Language switcher: links to the same page in the other language #}
<span class="theme-switch-button btn btn-sm btn-outline-primary navbar-btn rounded-circle" data-toggle="tooltip"
  {% if language == 'zh' %}title="Switch to English"{% else %}title="切换到中文"{% endif %}>
  <a id="lang-switch" href="#" style="display:flex">
    <i class="fa-solid fa-language"></i>
  </a>
</span>
<script>
(function() {
  var link = document.getElementById('lang-switch');
  var path = window.location.pathname;
  {% if language == 'zh' %}
  // Chinese -> English: remove the /zh/ path segment
  link.href = path.replace(/\/zh\//, '/');
  {% else %}
  // English -> Chinese: insert /zh/ before the page-relative path
  var pagePath = '{{ pagename ~ file_suffix }}';
  var idx = path.lastIndexOf(pagePath);
  if (idx > 0) {
    link.href = path.substring(0, idx) + 'zh/' + path.substring(idx);
  } else {
    // Fallback for root with trailing slash
    link.href = path.replace(/\/?$/, '/') + 'zh/';
  }
  {% endif %}
})();
</script>