http://www.matthijs.net/downloads/tools/translate.phps
To use this tool, copy-paste the text from your browser into a file called translate.php and put this into the root directory of your phpBB installation.
Edit the file with an editor of choise to set the configuration options (e.g. to and from languages and lang_main, lang_admin)
Update: New version supports 2.0.1 style translation files and fixes some little bugs.
Note: This translation tool does currently NOT support the translation of the FAQ. If anyone wants to add some scripting for that, I'll be pleased to insert it into the script and update the version. PM or email me about that, but be sure to use the official phpBB coding style.
NEWS:
Not supported by me, but nevertheless interesting:
NickInUse wrote: I've created another tool for beginning a new translation or updating an existing one, it can be found @ http://www.ofmy.info/ (phpBB2x board translation maintenance interface).
Does the lang_admin.php, lang_bbcode.php, lang_faq.php, lang_main.php files.