When exporting messages with the Search Exporter that contain multi byte characters these characters are replaced with "?" in the resultant PST file.
This is also an issue when using the Exchange Utility to unstub stripped messages.
Multi byte characters includes Japanese, Russian and Korean.
These utilities do not properly convert double byte unicode characters to ASCII.
CR 92599 has been created for this issue and will be addressed in a future release.
A workaround to the Search Exporter limitation is to use Recovery Manager for Exchange - Archive Manager Edition to export these double byte characters correctly.
This KB article has a video which will demonstrate how to add Archive Manager Mailboxes and restore to PST file via Recovery Manager for Exchange - Archive Manager Edition:
This is a link to a post in the Archive Manager community which will explain how to get a license for Recovery Manager for Exchange - Archive Manager Edition: