The default language functionality is not functioning as expected with the Polylang plugin.

choose-lang.php

replace (comment) this line in wp-content/plugins/polylang/frontend/choose-lang.php file (line 185)

$language = $this->options['hide_default'] && ( ( isset( $_SERVER['HTTP_REFERER'] ) && in_array( parse_url( $_SERVER['HTTP_REFERER'], PHP_URL_HOST ), $this->links_model->get_hosts() ) ) || ! $this->options['browser'] ) ?
        $this->model->get_language( $this->options['default_lang'] ) :
        $this->get_preferred_language();  // sets the language according to browser preference or default language
by

$language = 'fa';

protected function home_language() {
		// Test referer in case PLL_COOKIE is set to false. Since WP 3.6.1, wp_get_referer() validates the host which is exactly what we want
		// Thanks to Ov3rfly http://wordpress.org/support/topic/enhance-feature-when-front-page-is-visited-set-language-according-to-browser
		 $language = $this->options['hide_default'] && ( wp_get_referer() || ! $this->options['browser'] ) ?
			 $this->model->get_language( $this->options['default_lang'] ) :
			 $this->get_preferred_language(); // Sets the language according to browser preference or default language
		 $this->set_language( $language );
		
		 
	}

Replace By

protected function home_language() {
		// Test referer in case PLL_COOKIE is set to false. Since WP 3.6.1, wp_get_referer() validates the host which is exactly what we want
		// Thanks to Ov3rfly http://wordpress.org/support/topic/enhance-feature-when-front-page-is-visited-set-language-according-to-browser
		//$language = $this->options['hide_default'] && ( wp_get_referer() || ! $this->options['browser'] ) ?
			//$this->model->get_language( $this->options['default_lang'] ) :
			//$this->get_preferred_language(); // Sets the language according to browser preference or default language
		$language = 'pt';
        $this->set_language( $language );
	}



////LANGUAGE - function 
function custom_polylang_langswitcher() {
	$output = '';
	if ( function_exists( 'pll_the_languages' ) ) {
		$args   = [
			'show_flags' => 1,
			'show_names' => 0,
			'echo'       => 0,
		];
		$output = '<ul class="polylang_langswitcher">'.pll_the_languages( $args ). '</ul>';
	}

	return $output;
}

add_shortcode( 'polylang_langswitcher', 'custom_polylang_langswitcher' );