How to create a language pack
From Joomla! Documentation
The "J1.5" namespace is an archived namespace. This page contains information for a Joomla! version which is no longer supported. It exists only as a historical reference, it will not be improved and its content may be incomplete and/or contain broken links.
This page is tagged because it NEEDS REVIEW. You can help the Joomla! Documentation Wiki by contributing to it.
More pages that need help similar to this one are here. NOTE-If you feel the need is satistified, please remove this notice.
If someone has some time, please transfer/rewrite the information from the following reference into this page. Thank you.
Joomla! uses three language packages, one for the front end, one for the administrator and one for installation.
Many people find it helpful to use the translation manager extension.
Language naming convention in Joomla are language and ISO code. Use 2 letters when available. The list is available here.
This reference concerns country codes
Because language strings may change it is best to create your packages using either the nightly build or subversion.
Name of package should be: xx-XX //using your language's ISO code.
Locale: Information on available locales may be found here.
If, for any reason, ini or xml language files are edited manually with a text editor, they have to be saved with the encoding UTF8 NO BOM.
For practical information on the way to make core Translation files, please see forum: http://forum.joomla.org/viewtopic.php?f=11&t=254709.