Language switcher (content) does not work

I'm pretty sure at least one of the issues you are reporting is #1294946: Language detection based on session doesn't work with URL aliases.

AFAIK content language affects mostly the language fields are displayed in. Obviously if both UI and content language are configured to follow URL language the content language switcher will affect both fields and UI strings.

You can create a languageswitcher by yourself, here is a sample:

/**
 * Implements hook_block_info().
 */
function YOURMODULE_block_info() {
	$blocks = array();
	$blocks['languageswitcher'] = array(
		'info' => t('Customized Language Switcher'),
	);
	
	$blocks['toppages'] = array(
		'info' => t('Top Pages'),
	);

	return $blocks;
}


/**
 * Implements hook_block_view().
 */
function YOURMODULE_block_view($delta = '') {
	$block = array();
	global $language;
	
	switch($delta) {
		case 'languageswitcher':
			$block['subject'] = t('Switch your language:');
			$sql = "
				SELECT
					language,
					native
				FROM
					languages
				WHERE 
					enabled = 1
				ORDER BY
					weight ASC
			";
			$result = db_query($sql);
			
			$block['content'] = '<ul class="language-switcher-locale-url">';
			foreach($result as $row) {
				$block['content'] .= ($language->language == $row->language) ? '<li class="'.$row->language.' active">' : '<li class="'.$row->language.'">';
					$path = "";
					
					// $block['content'] .= arg(0);
					
					switch(true) {
						case (arg(0)=='taxonomy' && arg(1) == 'term'):
							// Für Terms
								$path = drupal_lookup_path('alias', 'taxonomy/term/'.(int)arg(2), $row->language);
							break;
						case (arg(0)=='node' && is_numeric(arg(1))):
							// Für Nodes
							$path = drupal_lookup_path('alias', 'node/'.(int)arg(1), $row->language);
							break;
						default:
							// Für Views und andere multilinguale Seiten
							$path = current_path();
					}
					
					if($path !== false ) {
						$block['content'] .= '<a href="/'.$row->language.'/'.$path.'">'.$row->native.'</a>';
					} else {
						$block['content'] .= '<a href="#" class="inactive">'.$row->native.'</a>';
					}
					
				$block['content'] .= '</li>';
			}
			$block['content'] .= '</ul>';
		
			break;
	}
	return $block;
}

Thanks mkweb!
You saved my life.

Updated mkwebs to just return it as array data, support table prefixes (required { }) and it supports links that don't use aliases.

function MYMODULE_get_languages() {
    $language_list = array();

    global $language;
    $language_default = language_default();

    // Force no-prefix when on other language pages
    // (Stops language prefix being applied on an alternate language page)
    global $language_url;
    $base_language_url = clone $language_url;
    $base_language_url->prefix = "";

    $sql = "SELECT language, native FROM {languages} WHERE enabled = 1 ORDER BY weight ASC";
    $result = db_query($sql);
    foreach($result as $row) 
    {
        $class = $row->language;
        $is_active = false;
        if ($language->language == $row->language) {
            $class .= " is-active";
            $is_active = true;
        }

        $path = "";
        if ($language_default->language != $row->language) {
            $url_options = array('prefix' => $row->language."/", 'absolute' => true, 'language' => $base_language_url);
        } else {
            // No prefix for default language
            $url_options = array('absolute' => true, 'language' => $base_language_url);
        }

        if (arg(0)=='taxonomy' && arg(1) == 'term')
        {
            // Terms
            $path = drupal_lookup_path('alias', 'taxonomy/term/'.(int)arg(2), $row->language);
            if ($path == false)
            {
                // Fallback to unaliased taxonomy
                $path = url('taxonomy/term/'.(int)arg(2), $url_options);
            }
        }
        else if (arg(0)=='node' && is_numeric(arg(1)))
        {
            // Nodes
            $path = drupal_lookup_path('alias', 'node/'.(int)arg(1), $row->language);
            if ($path == false)
            {
                // Fallback to unaliased node
                $path = url('node/' .(int)arg(1), $url_options);
            }
        }
        else
        {
            // Unknown
            $path = current_path();
        }

        $language_list[] = array(
            "title" => $row->native,
            "url"   => $path,
            "class" => $class,
            "is_active" => $is_active,
        );
    }
    return $language_list;
}

After enabling the customized language switcher block, using code in #4 and #7, The switching does not apply the default language recognition settings. I have English as default site language, and that does not use the "/en/" url prefix.
Thus I get the error:
The requested page "/en/about" could not be found.