Why are international characters in notification email subjects are being replaced with '?'
This happens if the system encoding is not the same as the JIRA encoding (by default UTF-8). System encoding can be seen in Administration -> System -> System Info, and JIRA encoding can be seen in Admin -> Global Settings -> General Configuration.
If there is a discrepancy, this can be fixed by setting the system encoding with a command-line option (-Dfile.encoding=utf-8) when starting JIRA. Eg. with Tomcat (JIRA Standalone), set the following before running the startup script:
See JRA-5176 for more details.