What it says on the tin. I basically took the legion naming system from Rome Total War:Barbarian Invasion (where if you recruited a first cohort of Comitatensis, it would have the subtitle "Legio X Italia", or whatever region you recruited it from), and applied it to EBII. Only here I applied it to every unit (barring general's bodyguards, ships and mercenaries).
Obviously, its not that historically accurate to have a unit of Sphendonatai or whatever running around labelled "Legio XV Attike", but since its impossible to edit the names of individual units, this is the next best thing to keep track of them.
I've attached an image of how it looks below, and the relevant files you need. Only two files have been edited: descr_regions.txt and export_descr_unit.
Make backups, copy and paste into your ebii folder, and make sure to delete the map.rwm, or else the changes won't take affect.
If you guys are interested in making edits of your own, I learned how to do this from this tutorial:
Even so, a legion was a military unit/formation/battlegroup of around 3000 men, if i recall correctly? The equivalent of a modern brigade. Let it be. Better than nothing. Does this apply to all units or only the ones of your faction?