Suggests (to me) that you might create a new server with the DE settings and restore the database there.
If you don’t like the safety of being sure they the new site works before you break the old one, and moving to a new os when your old one is likely approaching end of life, you could probably also make a backup, delete the postgres files, change the locale, rebuild to create the new database, and then restore the backup.
EDIT: I speak only English and am not especially expert at the intricacies of locales, but your concern about searches working correctly seem on point to me.