Databases lost after Postgres upgrade from 11 to 12 This is not my first upgrade. In the past I upgraded from 9 to 10 and 10 to  11. I thought that I wrote everything down the last time but apparently not. In verion 11 my databases are in nl_BE.utf-8:
                              List of databases
   Name    |  Owner   | Encoding |   Collate   |    Ctype    |
-----------+----------+----------+-------------+-------------+
 glewlwyd  | glewlwyd | UTF8     | nl_BE.utf-8 | nl_BE.utf-8 |
 postgres  | postgres | UTF8     | nl_BE.utf-8 | nl_BE.utf-8 | 
 prda      | postgres | UTF8     | nl_BE.utf-8 | nl_BE.utf-8 |
 template0 | postgres | UTF8     | nl_BE.UTF-8 | nl_BE.UTF-8 |
 template1 | postgres | UTF8     | nl_BE.utf-8 | nl_BE.utf-8 |

(5 rows)

After the installation (apt-get install postgresql-12) I modify the /etc/postgresql/12/main/postgresql.conf and change the en_US.UTF-8 into nl_BE.UTF-8. In the previous upgrades I copied this file to my data directory but 12 does not seem to like that. The port number of the 11 and 12 version differ.
When I create the database with /usr/lib/postgresql/12/bin/initdb --locale=nl_BE.UTF-8 --pgdata=/srv/postgres/data/12 the new instance is created but the databases are in en_US.UTF-8. The /usr/lib/postgresql/12/bin/pg_upgrade -b /usr/lib/postgresql/11/bin -B /usr/lib/postgresql/12/bin -d /srv/postgres/data/11 -D /srv/postgres/data/12 ends without an error.
When I connect to the instance my own databases are not there. Must be because the locale is not correct. I also changed the locale (export LANG=nl_BE.UTF-8) of my shell but this did not help. What could I have done wrong?
I followed the comment from @Colin 't Hart and this solved the problem. With pg_upgradecluster -v 12 11 main /srv/postgres/data/12 the upgrade was done. I changed the port to 5432 and restarted it. Now my applications are using the version 12 database.