Per the Avaya upgrade guide, you should get this to release 6.3.22 first. I would spin up 6.3 in VMWare, restore the 5.2.1 translations there and then move to 8.1.
Plan on having to recreate your CM admin users and TLS certs. Your XLNs should come across just fine.
If you are using SES, you've got a whole 'nother set of work to do (check the upgrade guide for those notes about ASM).