There are quite a few spots where it's referenced. I recommend using a program that can set the folder as a workspace and search through all the text files. Notepad++ works for this, it's what I use. With it I can just search the entire events or the entire common folder for add_trait = berserker and find all the places where it's referenced. I think in all the cases it requires either north germanic cultural heritage, or the north germanic religion; plus it requires "warmonger" tenet in your religion.

It gets added in the jomsvikings event to their leader in events\dlc\fp1\fp1_jomsvikings_events.txt.

Asatru mysterious stranger events can spawn them, in events\dlc\fp1\fp1_yearly_events.txt

The one the player usually sees to learn berserker from other berserker event that can fire is in events\dlc\fp1\fp1_yearly_events_oltner.txt.

Knights can get it in common\combat_phase_events\00_knight_phase_events.txt under knight_become_berserker.

The character templates that generate AI characters can make them in common\scripted_character_templates\00_pool_repopulation_character_templates.txt

There's a varangian template that can get it in common\scripted_character_templates\07_ep3_character_templates.txt

There's a bunch of templates in common\scripted_character_templates\04_ep2_accolade_character_templates.txt that can get it.

pool_character_development_effect incommon\scripted_effects\00_courtier_guest_management_effects.txt can add it to courtiers.

You can either edit the initial event and/or the templates to add your own religion and culture group to the list, or make your own custom events to add the trait. There isn't a quick change to change all of them, as they all individually check for north germanic heritage and warmonger tenet instead of pointing to a custom trigger.

As far as localizing the trait, you can edit the trait and add triggers to adjust the name and description based on your culture and/or religion. For an example of this, you can look at the "Devoted" trait. It has different names and descriptions based on religion and the owner being an adult or child.