Greetings and Salutations from the Imperial Splendour crew. We just discovered this, and thought we'd share it with all of you.
This tutorial requires 2 programs: the ESF Editor and the PFM (or DB Editor, your choice).
1) Open PFM and open the localisation. In English versions of the game, it's in patch_en.pack (in the data folder). Extract the localisation to your data folder. Now create a new pack with the PFM. Add the localisation that you just extracted. Save as whatever you want your pack to be called. I personally recommend something like patch_xxx_text.pack. That way everybody knows it isn't in mod format and they also know that it's the text localisation. Change your pack type to movie. Save it again.
2) Open your localisation. Scroll down until you see a bunch of entries like unit_regiment_names_localisation_lookup_unit_name_euro_infantry_units_001 through whatever it goes to. Clone a number of them, however many you need. For this first part I'd just go with a small number, say 4, until you know that you did everything right.
3) Rename your cloned entries from unit_regiment_names_localisation_lookup_unit_name_euro_infantry_units_001 (or whatever number it is) to something like unit_regiment_names_localisation_lookup_unit_name_russia_infantry_units_001 (the format of this part doesn't really matter. You can really call it whatever you want, so long as you remember what it is). Now change the next column to the name of the unit that you want displayed. For me, I went with these Russian units, in this order: Rostovski Regiment, Kievski Regiment, Vologdski Regiment, Sibierski Regiment (each one of these corresponding to a different russia_infantry_units_00x entry).
4) Save your new pack.
5) Open the ESF Editor. Open the startpos.esf in the campaigns/main folder (make sure you have a backup of it before you start working). Scroll until you find CAMPAIGN_STARTPOS/CAMPAIGN_ENV/CAMPAIGN_MODEL/WORLD/FACTION_ARRAY/FACTION_ARRAY/FACTION. Find the faction you want to change. In my example, it's Russia. Just keep clicking the FACTION entries until you find what you want.
6) Continue to LAND_UNIT_NAME_ALLOCATOR/LAND_UNIT_NAMES_MAP/LAND_UNIT_NAMES_MAP. You'll notice that by clicking on these entries, it displays a number. The numbers are 0-22, although not in that order. These entries correspond with the unit classes as defined in the unit_class table in main.pack, and the order in which they are in the table. So #0 would be artillery_fixed and #12 would be general. I'll provide a list of the numbers and their corresponding unit classes at the end of this post so you don't have to keep checking the main.pack.
7) Find the number that corresponds to the unit class that you want to change. In my case, it was infantry_line, which is #18. Further expand UNIT_CLASS_NAME_ALLOCATOR/UNIT_CLASS_NAMES_LIST/UNIT_CLASS_NAMES_LIST. You'll see a bunch of entries. Each one of these, in order, contains a pointer to the localisation entry of each successive recruited infantry_line unit.
8) Expand UNIT_CLASS_NAMES_LIST and open/click on CAMPAIGN_LOCALISATION. You'll see an entry that will hopefully be unit_regiment_names_localisation_lookup_unit_name_euro_infantry_units_001. Change that to the new localisation that you defined, which in my case above was unit_regiment_names_localisation_lookup_unit_name_russia_infantry_units_001. Do this again for the next 3 units (incrementing the pointer, of course). Save and exit.
9) Boot up the game and when you recruit a new Line Infantry unit as Russia, you'll see whatever name you gave it. In my case, it was Vologdski Regiment. The reason why it doesn't start at #1 is because there are already Line Infantry regiments on the board. Which it doesn't re-name. That's a different entry in the startpos, which I might write a tutorial for later.
Good luck and have fun.
ps - Here's that breakdown of unit classes I promised:
Spoiler Alert, click show to read: