psql needs to be restarted to see freshly added locales.

roles/gitea/tasks/setup.yml

@@ -64,6 +64,13 @@
   locale_gen:
     name: en_US.UTF-8
     state: present
+  register: locale_gen_result
+
+- name: Force-restart PostgreSQL after new locales are generated
+  service:
+    name: postgresql
+    state: restarted
+  when: locale_gen_result.changed
 
 - name: Create psql git user
   become: true