For updating regional name.
1. It does not depend on zone.
2. If train connect to different regional language states then departing station language will be considered.
3. If train connect a hindi region state with a non hindi region state then that language will be updated in both directions.
4....
more... Himanchal Pradesh, Hariyana, Delhi, UP, Uttarakhand, Bihar, Jharkhand, Chhattisgarh, MP and Rajasthan having no other regional language except Hindi so trains connecting these states can be updated with using secondary languages.
5. So for regular practice we can use
Delhi, Hariyana, Himanchal- Punjabi
UP , Uttarakhand, Rajasthan, Bihar- Urdu
Jharkhand- Bengali or Urdu
MP, Chhattisgarh - Marathi.
Note- these languages should be considered only if train is running between given states only, if train is bound to any other state then that regional language will be considered foe both directions.