Take a look in /js/lightbox.js
at line 29-31:
LightboxOptions.prototype.albumLabel = function(curImageNum, albumSize) {
return "Image " + curImageNum + " of " + albumSize;
};
Translate the strings in any language / text you want to.
Based on this version: https://github.com/lokesh/lightbox2/blob/master/js/lightbox.js