Add language filter at admin/structure/block/block-content I'd like to add a language selector in the form that filters.
I've added it though a hook form alter:

$form["#info"]["filter-language"] = [
  "operator" =&gt; "type_op",
  "value" =&gt; "language",
  "label" =&gt; "Block language",
  "description" =&gt; ""
];
$language_options = [];
foreach (\Drupal::languageManager()-&gt;getLanguages() as $lang_code =&gt; $language) {
  $language_options[$lang_code] = $language-&gt;getName();
}
$form['language'] = [
  '#type' =&gt; 'select',
  '#title' =&gt; t('Block language'),
  '#options' =&gt; $language_options,
];
That page is a view.

That means, you can simply edit the view and add fields, filters and so on in the UI. You can get there by finding the contextual links icon and clicking "edit view" or by going to "/admin/structure/views/view/block_content/edit/page_1".