RPGMaker Trans development had essentially stopped as of a year ago, because JapeTrans is pretty much a from the ground up rewrite (there was a post on this on the website, but I'll admit I've been quite lax in communicating progress on JapeTrans). And RPGMaker Trans has never claimed to be completely generic. In fact, there's bits of VPM that RPGMaker Trans cannot touch (the scene names in the event viewer). JapeTrans is quite a lot more generic because you can specify which contexts you want to translate (dumping everything is rarely a good idea, because modifying file name strings is unlikely to end well). There's also a plan to be able to add plugins to handle preprocessing strings - mostly for the purposes of making things easier to parse for translators, but it could be used to support games which cipher the text inside themselves (OneOne1 springs to mind,), but that's in its infancy.
XML would be pretty easy to add as a dialect, but it's not a high priority - the reason being that it's verbose, and isn't particularly friendly to translators. Basically the more stuff a user has to write, the less likely I'm to add it. Tex-like was chosen because it's concise and pretty clear what's going on in it when you start defining your own macros i.e. \macro{text} is applying a macro to some text, or \macro for a no-argument version. None of the alternative dialects I've seen are anywhere near as concise or clear (e.g. XML is <macro>text</macro> or <macro/>). And yes, do remember that I'm only using Tex for it's syntax, and not implementing all of Latex, so it's substantially simpler.
I would note that in writing this post, I've also realised that I'm not sure this type of thing is even possible in RST, so perhaps I should not do that one at all...
Anyhow, perhaps best to wrap up on the discussion of possible future JapeTrans features because I fear this is getting far too off topic. I'll keep the discussion of JapeTrans to only things which are relevant to VPM.