maybe I found reason:
In 

src/worker/workerinstallprogress.cpp 
void WorkerInstallProgress::setTransaction(Transaction *trans)  <- this function 

the m_trans->locale() is Empty, so it's not set a codec for locale.

It's my fault, need to setting a local to transaction. closed.

Seems to me the API should maybe default initialize locale to whatever is in QLocale of the application.
Having to specify the locale explicitly is a fairly avoidable pitfall when using libqapt.

If you care enough to propose a change we could improve that ;)