Set default language with Wordpress Polylang plugin.

wordpress-polylang-plugin-default-language.php

<?php

add_filter('the_content', 'lngSetDefaultLanguageAsPortuguese');

/**
 * @param string $content
 * @return string
 */
function lngSetDefaultLanguageAsPortuguese($content)
{
    /** WP_Post $post */
    global $post;

    if (!function_exists('pll_get_post_language')){
        return $content;
    }

    if (!function_exists('pll_set_post_language')){
        return $content;
    }

    /** @var string|bool $postLanguage */
    $postLanguage = pll_get_post_language($post->ID);

    if ($postLanguage === FALSE){
        pll_set_post_language($post->ID, 'pt');
    }

    return $content;
}