When JIRA (or Confluence it seems) sends out a trackback ping, it converts non-ASCII characters to ?'s.

To replicate, set up two JIRA Standalone (hsqldb) instances at http://localhost:8081/jira1 and http://localhost:8082/jira2. Make sure to correct their base URLs, and make the issues anonymously visible so pings are sent. Create an issue with a non-ASCII summary (I used Картинки from google.ru). Wait for the ping to appear in the linked issue. It appears as in the screenshot.

Capturing the ping POST with ethereal shows:

POST /jira1/rpc/trackback/TP-13 HTTP/1.1
Content-Type: application/x-www-form-urlencoded
User-Agent: Jakarta Commons-HttpClient/3.0-rc3
Host: localhost:8081
Content-Length: 272

url=http%3A%2F%2Flocalhost%3A8082%2Fjira2%2Fbrowse%2FTP-16&title=%5BTP-16%5D+%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F&blog_name=JIRA%3A+Test+Project&excerpt=http%3A%2F%2Flocalhost%3A8081%2Fjira1%2Fbrowse%2FTP-13+null
HTTP/1.1 200 OK

Those are definitely question marks:

python -c 'import urllib; print urllib.unquote("url=http%3A%2F%2Flocalhost%3A8082%2Fjira2%2Fbrowse%2FTP-16&title=%5BTP-16%5D+%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F%3F&blog_name=JIRA%3A+Test+Project&excerpt=http%3A%2F%2Flocalhost%3A8081%2Fjira1%2Fbrowse%2FTP-13+null")'

url=http://localhost:8082/jira2/browse/TP-16&title=[TP-16]+????????????????????????????????&blog_name=JIRA:+Test+Project&excerpt=http://localhost:8081/jira1/browse/TP-13+null