When using the native tool the Search Exporter to export messages with double byte characters the characters show up as ??? in the resultant PST file.
This is described in SOL71671
This issue has been captured in CR92599
A workaround is to use Recovery Manager for Exchange - Archive Manager Edition as shown in this video:
This is a link to a post in the Archive Manager community which will explain how to request a license for Recovery Manager for Exchange - Archive Manager Edition