About the one file : maybe you could put the localization in the DB ? Load it from a file (to ease translation, management and so on), but when it's done all call are made to the DB. It should be faster than including a php file, no ?
About the keep up : I haven't been here long enough to say, but from my fresh point of view the multiples files are an issue to keep up. For example, there is a french up to date pack (I think). I disagress seriously with several choices of translation, so I want to edit it. We (french) have the chance of having the languages files in a wiki at punbb.fr, so I already corrected most of it.
But for the remaining ones (including banning HTML entities; and using only utf8 ? which by the way could be tricky since it's PHP file, not text... I don't know, I will see when I try it) I have to check a dozen files, correcting, re-encoding, checking the choices I've made in others files to stay coherent, and so on.
So I would make this "new pack" avalaible if anyone is interested, and I will certainly keep-up to the new PunBB versions when I update my own forum(s), but it will mean more maintenance, more work from me, and more work for the people interested. In the end, it could explain why some poeple make their owns mods on their own and don't share.
Well, with SVN I maybe could create a patch for it (I don't know if I can limit the scope of a SVN patch, to not include others modifications or hacks I've down elsewheres), but most people are afraid of SVN and won't use it.
).