main log encoding problem

I'm using postgresql 9.1.3 from debian squeeze-backports with zh_CN.UTF-8
locale, i find my main log (which is
"/var/log/postgresql/postgresql-9.1-main.log") contains "???" which
indicate some sort of charset encoding problem.
But error messages related to pgsql is fine, only other system messages
have this problem, for example:

2012-05-19 16:06:12 CST ??: ?????????? 2012-05-19 16:06:10 CST
> 2012-05-19 16:06:12 CST ??: ???????????
> 2012-05-19 16:06:12 CST ??: ???autovacuum
> 2012-05-19 16:06:12 CST ??: ???????
> 2012-05-19 16:07:16 CST 错误: 角色"postgres" 已经存在 (in english: Error: role
> "postgres" already exists)
> 2012-05-19 16:07:16 CST 语句: CREATE ROLE postgres;
> 2012-05-19 16:07:16 CST 错误: 语言 "plpgsql" 已经存在 (in
> english: Error: language "plpgsql" already exists)
> 2012-05-19 16:07:16 CST 语句: CREATE PROCEDURAL LANGUAGE plpgsql;
> 2012-05-19 16:08:23 CST ????: ?? "huangyi" ???
> 2012-05-19 16:08:52 CST ????: ?? "huangyi" ???
> 2012-05-19 16:09:01 CST ??: ???????(zlfund)????????(huangyi) ???
> 2012-05-19 16:09:01 CST ????: Peer authentication failed for user "zlfund"
> 2012-05-19 16:09:34 CST ??: ???????(zlfund)????????(huangyi) ???
> 2012-05-19 16:09:34 CST ????: Peer authentication failed for user "zlfund"

I guess it has something to do with packaging problem rather than
postgresql itself, but it would be great if you can give me some clue where
the problem might be.

My best regards.
Yi Huang.