I received an answer from Sony about 4.3 update in July 2014. Here is the answer:
Thank you for contacting Sony Xperia Support.
We are sorry for the delayed response to your email.
With regards to your inquiry, we inform you that at the moment the latest available software version for your phone model Sony Xperia C is version of Android 4.2 Jelly Bean.
The latest available version of the software: 6.2.B.1.96
Unfortunately, at the moment we have no information regarding the release date of updates on Sony Xperia C to version Android 4.3.
You can find detailed information on our website, in the "Software", "Check for updates" and then select your phone model from the list: http://www.sonymobile.com/ru/software/
Should you require any further assistance please contact us on 8 800 100 8022. For more information on our products, services and support please visit www.sonymobile.com
Please be aware that if you do not respond within 7 days of this email your query will be automatically closed. Any further communications will not reach our support and a new query will need to be raised.
They said, that the latest software version is 6.2.B.1.96
I checked for updates, and it said that no updates are avaible. Im in Ukraine. Check for updates and tell me if you have one!!!