How can I set default language for trac. There's nothing about i18n in trac.ini
The stable release of trac, 0.11.X does not have support for localization. It is added in the upcoming 0.12 which is currently under development. Do you have 0.12 or did you download "latest stable" which is 0.11? In that case you will have to upgrade to get localization support.
The trac site itself appears to be running 0.12 as it is localized.
Trac default language