I am afraid, there is no “Found start of mail merge region XXX without matching end” error in 15.4, we have made sure by checking the code. However, it was added in 17.12 version.
Also, you have not attached a sample document to this thread. Please also ZIP and upload your sample input Word document here for further testing. We will then investigate the issue on our end further and provide you more information.
Also, considering the possibility of adding this as a new feature, could you please also explain if you want to only translate just mail merge related messages or all Aspose.Words messages?
We are not going to translate the messages, but may consider changing exception types to be more specific and contain MM specific parameters (like RegionName). That way, our customers could catch them and re-throw using their own translated messages if needed. But, this is only feasible if you are fine with only MM exceptions to be changed that way. It is currently not possible to change all Aspose.Words exceptions.
Thanks for your cooperation.