Check if the .po file for your language already exists.
Download the .po file you need and convert it to machine-readable .mo file. You can do this with:
Using poedit software
Using gettext command: msgfmt
<LANG>.po -o <LANG>.mo
Using any other gettext software
Upload it to path
/usr/langs in your Typecho. If that folder doesn't exist, you can create it before upload it.
Go to control panel of Typecho to switch to your language. It often locates at http://your-blog-url.com/admin/options-general.php
Fork this repo and clone to local.
Start your translation road by using PoEdit or other gettext software. The message.pot file contains all recent translation strings, but it is in Chinese, so you might want to start with the English translation file en_US.po and update the missing messages from messages.pot.
Push update to your forked repo, and then pull a request to offical languages repo: named typecho/languages.
Also please see http://docs.typecho.org/translate/start