I've looked for some updated translation instructions, but most of it is obsolete. Instructions for 5.x version, latest language files for Slovenian dated in late 2007...
So until I find out the proper way to contribute with my translations, I'll simply post them here (and keep them updated as I translate further)

Version 1.1
Approx. 15% translated (mostly main user Web GUI, tabs, buttons, actions)
ZCS version: 8.0.2
Download: slovenian_sl_SI.rar
CRC check: baa430ec
SHA-1 check: 9abdd8fb2c606a7c201068a42348339b3376e646

Installation instructions:
Simply copy these files to:
You will not overwrite ANY existing files.

Make sure files permissions are 644 and are owned by zimbra user and zimbra group.

Then restart Zimbra WEB server.
If you do not know how, simply restart all Zimbra services as zimbra user:
zmcontrol restart
After restart, reload Webmail browser and you can select Slovenian language under Preferences.
Also, as admin you can push Slovenian language as default for all, existing and new users via COS.
Not translated parts are - of course - in english.

Any ideas how to contribute in official release, please, welcome!