file installation/controller/setdefaultlanguage.php
InstallationControllerSetdefaultlanguage::execute

Form field "installLocalisedContent" is not checked in the code.
A category and articles for each language are always created even if the related toggle is turned off.

if (!$error)
{
    $tableCategory = $model->addCategory($siteLang);

    if ($tableCategory === false)
    {
        $app->enqueueMessage(JText::sprintf('INSTL_DEFAULTLANGUAGE_COULD_NOT_CREATE_CATEGORY', $frontend_lang));
        $error = true;
    }
}

if (!$error)
{
    $categoryId = $tableCategory->id;

    if (!$model->addArticle($siteLang, $categoryId))
    {
        $app->enqueueMessage(JText::sprintf('INSTL_DEFAULTLANGUAGE_COULD_NOT_CREATE_ARTICLE', $frontend_lang));
        $error = true;
    }
}