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One thing to note when choosing a server-side language to learn: if you don't self-host, you may find that many clients register webspace themselves (as has often been my experience), and significantly more web hosts support PHP than ASP.net.
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Sorry to butt in, because I'm no expert, but if this is a long-term project, maybe you should think about server-side XML translation. Then you'd only have to worry about it once. I've briefly done some XSLT, but after learning some PHP methods, it made it a lot easier to integrate my XML with the rest...