we have a long-term problem with encoding in email subjects, where characters like "šáěíéščřý" are not shown correctly. Until now, we have been changing the subjects to use basic ascii letters. But now we are expanding to russian customers, and with azbuka (cyrilic) letters we can no longer do this.
Please check Apache Sling Request Parameter Handling configuration for the Default parameter encoding" for your instance. Generally this encoding is used to display or send the data [which can include special characters as well]. If you are using the AEM default Day CQ Mail Service checking this would be helpful.