I propose this fix :

Add in each language a new label :
22a23

'legend': 'Moving $1',

This new label can be initialized with the content of label 'move'
Here is the french translation :

'legend': 'Déplacement de $1',

Then in openDialog() function, we can set the legend :
571a572,577

var message = mw.message(
        sites.length > 1 ? 'move' : 'legend',
        sites.length > 1 ? '' : sites[0]
);
$( '#sitelink-form legend' ).empty().text( message );

And please change this french label to print the sitelink $2 :

'success': 'Le lien interlangue $2 a été déplacé avec succès vers l\'élément $1.',

Here is the diff file of my proposed changes