i18n & variables manipulation

Hi all =)

I’m working on the i18n of my youtube shortcode plugin and I face a specific issue: I have put the generated player inside a variable and if the user forgot to enter the ID of the video then this variable contains an error message. This part looks like this:

if (empty($id)) {
   $generated_output = '<span class="error-pxsol_youtube">You must set the ID of the YouTube video.</span>';
};

The question: how do I included the “_e('Some texts here', 'domain');” kind of code into this output message (I tried 3 ways of coding it but the PHP part seems to be treated as text)?