__() does not seem to work

Hello,

I have some problems with translating my plugin because the __() function does not seem to work as it should.

Here is an example (radically simplifyed version of my plugin’s class):

class MyClass
	{
	var my_var;

	function MyClass()
		{
		add_action('init', array($this, 'plugin_lang_init'));
		add_shortcode('plugin_output', array($this, 'html_output'));

		// Here will also be separated strings concated together
		// e.q. __('first_str').'<br />'.__('another_str') and so on.
		$this->my_var = __('My first string', 'my_plugin');
		}

	function plugin_lang_init()
		{
		load_plugin_textdomain('my_plugin', false, dirname(plugin_basename(__FILE__)).'/languages/');
		}

	function html_output()
		{
		$output = ''
			.'<div>'.$this->my_var.'</div>' // translation DOES NOT work
			.'<div>'.__('My second string', 'my_plugin').'</div>'; // translation works perfectly

		return $output;
		}
	}

As shown above: if the __() is stored inside the class property, translation does not occur.

Any ideas why this isn’t working? And what would be the alternatives?

Thank you.