StatusNet (homepage) is an open source microblogging server with full i18n support. While offering functionality similar to Twitter, StatusNet seeks to provide the potential for open, inter-service and distributed communications between microblogging communities. Enterprises and individuals can install and control their own services and data. The code is released under the Affero GPLv3+ license.
Siebrand coordinates the localisation update commits of StatusNet. Use support for i18n/L10n related questions. Use the StatusNet bug tracker for other issues, or when referred there.
Some notes on local terms and jargon in StatusNet are available on translatewiki.net to aid translators. Documentation and help pages are being drafted (as at June 2010) on StatusNet.
If you're more comfortable using an offline editor, set the correct user language and export translations to a .po file.
Note that .po files exported from translatewiki contain extra information to support round-tripping correctly. They cannot be used elsewhere. Submit them back through translatewiki.net for merging and normalization. You need the "offline" user right; be sure to request it, or you will not be able to use the submit interface.
If using POedit, be sure to use version 1.4 or later -- earlier versions will corrupt the .po files.