translate a variable

translate-variable.php

<?php
	$url = get_post_meta( $post->ID, '_project-url', true );
		if ($url != '') {
		$link = sprintf( __( '%s', 'wpp-portfolio' ), esc_url( $url ) );?>
		<p><?php echo $link;?></p>
	<?php }
?>

The short answer here is that you cannot make a variable that is obviously translatable like this. The reason is because an auto-generated .pot file will only include "%s" as the translatable text and a human translator will come along and ask, "What does %s mean?"

Now, that said, this can properly translate your passed variable, but it will only work if the translation file has a translation available for whatever content is passed in. The way to make this happen is to manually edit your generated .pot file to include all known values for %s so that a translator can translate them in the .mo they return to you. Of course, if you knew all of the possible variables for $url, you probably wouldn't be doing this in the first place :)

Here's a fork of your code sample. The first code sample is a simpler way of writing what you wrote and maintains the translation function. The second sample just removes the internationalization (since I suspect you won't know all possible values for the user-provided $url and therefore can't possibly include them in your .pot file to make translation practical)