Add i18n support to api.drupal.org

is it possible for me or anyone to request a sandbox of api.drupal.org?

Yes, the new version of the "API" module comes with a full demo that you can install locally: https://git.drupalcode.org/project/api/-/tree/2.x/demo?ref_type=heads

This matches 99% what's on api.drupal.org so you can play with it.

My biggest worry, already mentioned above, is the maintenance of these translations. This is a huge endeavor, there are thousands and thousands of pages, blocks, etc and sometimes it's all a single block with comments and markup embedded.

Markup is stored in the database like this (for this tiny snippet https://api.drupal.org/api/drupal/core%21modules%21action%21action.modul...):

<pre class="php"><code><span class="php-keyword">function</span> <span class="php-function-or-constant"><a href="/api/drupal/core%21modules%21action%21action.module/function/action_entity_type_build/11.x" title="Implements hook_entity_type_build()." class="local">action_entity_type_build</a></span>(array &amp;<span class="php-variable">$entity_types</span>) {
    <span class="php-comment">
    /** @var \Drupal\Core\Entity\EntityTypeInterface[] $entity_types */</span>
    <span class="php-variable">$entity_types</span>[<span class="php-string">'action'</span>]-&gt;<span class="php-function-or-constant"><a href="/api/drupal/11.x/search/setFormClass" title="Multiple implementations exist." class="local">setFormClass</a></span>(<span class="php-string">'add'</span>, ActionAddForm::class)
        -&gt;<span class="php-function-or-constant"><a href="/api/drupal/11.x/search/setFormClass" title="Multiple implementations exist." class="local">setFormClass</a></span>(<span class="php-string">'edit'</span>, ActionEditForm::class)
        -&gt;<span class="php-function-or-constant"><a href="/api/drupal/11.x/search/setFormClass" title="Multiple implementations exist." class="local">setFormClass</a></span>(<span class="php-string">'delete'</span>, <span class="php-string">'Drupal\\action\\Form\\ActionDeleteForm'</span>)
        -&gt;<span class="php-function-or-constant"><a href="/api/drupal/11.x/search/setListBuilderClass" title="Multiple implementations exist." class="local">setListBuilderClass</a></span>(<span class="php-string">'Drupal\\action\\ActionListBuilder'</span>)
        -&gt;<span class="php-function-or-constant"><a href="/api/drupal/11.x/search/setLinkTemplate" title="Multiple implementations exist." class="local">setLinkTemplate</a></span>(<span class="php-string">'delete-form'</span>, <span class="php-string">'/admin/config/system/actions/configure/{action}/delete'</span>)
        -&gt;<span class="php-function-or-constant"><a href="/api/drupal/11.x/search/setLinkTemplate" title="Multiple implementations exist." class="local">setLinkTemplate</a></span>(<span class="php-string">'edit-form'</span>, <span class="php-string">'/admin/config/system/actions/configure/{action}'</span>)
        -&gt;<span class="php-function-or-constant"><a href="/api/drupal/11.x/search/setLinkTemplate" title="Multiple implementations exist." class="local">setLinkTemplate</a></span>(<span class="php-string">'collection'</span>, <span class="php-string">'/admin/config/system/actions'</span>);
}

Then, on pages like this https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Entity%21...
translating certain bits might be wrong, as you might be translating code, like "class" or "implements".

For example, the content in the tables https://api.drupal.org/api/drupal/globals/11.x, or https://api.drupal.org/api/drupal/elements/11.x can't barely be translated.

Per #10 and #11, closing this. The source code is rendered in the language that the code is written in.