Just finished updating the Tagalog translations of the shadow package and sent the tl.po to Tomasz, the upstream maintainer. I'll try to get the updates to him much earlier since he takes the time to send update email to all translators ahead of a release.
d-i is definitely a moving target and I have to keep on my toes to keep the tl translation in sync. It is easier to do this daily since the updates are usually a few strings a day, and it is much more manageable to do that than all in one go. It would also make things easier if there were more translators who are dedicated to maintaining translations for even just one program each to distribute the load of translation. But at the moment, it's wishful thinking. The drawback to this is that the style of translation will vary with each translator. Someone should spend time writing up a style guide for translation in Tagalog as well as an updated glossary of technical terms with their suggested translations.