class.upload.php is a powerful and mature PHP class to manage uploaded files, and manipulate images in many ways. The script is available under a GPL license.
If you want to fill an image with a watermark, the best is probably to create a large watermark, that you will apply on your image, as an overlay. You can duplicate your watermark in a large image, something like this
As for the translations, have a look at this page. You can use this template