New language? Or want to help?
If you want to help with the translation of TYPO3 into multiple languages the first step for you is to subscribe to the translator mailing list here:
lists.netfielders.de/cgi-bin/mailman/listinfo/typo3-translators
All communication to translators about deadlines, features, changes etc. coming from the core team will be announced there. Also, that list can be used for discussions among translators themselves of course.
If you want to help with the translation of an existing language in TYPO3 you need to introduce yourself on the mailing list and tell how and why you want to help. If the existing translators for the language can accept your help they will tell you the URL of the translation server and which username/password you can use to login for that language.
If you want to add another language to TYPO3, then write to kasper about it, tell him your a) username, b) the charset needed c) the language key you would suggest (like "de" for germany or "it" for italian - 2 chars!) and d) tell him you read this page and knows how to proceed.
Before you write please be aware that it's a huge task. A minimum requirement is that you are willing to maintain the core translation and the most common and popular extensions as well. Further you should look out for assisting translators who can help you do it.
Translating extensions
I would like to describe the basic process of translating TYPO3 (core and extensions).
First of all, labels for any component is located in XML files prefixed "locallang" and found in extensions. The translations of the labels can either be included in the same files or found in external files, distributed as a language pack (recommended)
Language packs are basically locallang-XML files located in a corresponding folder in typo3conf/l10n/[language key]/
Language packs are downloaded and updated through the Extension Manager according to which extensions you have installed.
Contributing to a language pack is currently done through the "translation server". There is an extension called "llxmltranslate" which is designed to translate extensions on the system.
In the future contribution of translations will be possible from any local system where a person chooses to install the "llxmltranslate" extension and has translated labels for any extension, either core or third-party. The Extension Manager will be able to "commit" a translation to the translation repository "inbox" in TER from where it can be merged into the official language pack by the official translator team.
The philosophy of that is that extensions are translated by anyone in the context where a user needed it - on his own system! And after translating a single click of a button allows him to contribute the translation to the community.
