There are many active translations out there of the PHP documentation. Some lanaguages are being maintaned by a group of translators (eg. the German), some are one person projects (eg. the Japanese). There are quite many things for translators to know, though these are simple.
Starting a new language translation comes down to the following simple steps now.
Consult the mailing list to see if the translation is already in progress. If it is, disregard the following and coordinate with the other translators on the list.
Ask for a CVS account. Mention, that you would like to start a new translation.
Create a sudirectory with the appropriate language code, or ask for help about how you can do this on the list. Appropriate languages codes can be found at http://www.unicode.org/unicode/onlinedat/languages.html. See the CVS chapter about how to add a file or directory.
Copy en/language-defs.ent and en/language-snippets.ent to the new directory and translate the contents of them.
Copy over files from the English tree and start to work on them, (do not check in untranslated files) and rerun configure each time you add a file. See the section about revision tracking for help about how can you ease your work of tracking the English versions, and your languages versions.
Some important issues to consider when building a new translation:
Can you set up and translate the whole manual yourself? If not, can you set up a team to work on the language?
Have you made sure that your language or glyph (lettering, font, or characters) is supported by the DocBook stylesheets? Please ask on the mailing list if you don't know.