BUG #15038: Upgrade 8.4 to 9.2 - LC_Collate differs

The following bug has been logged on the website:

Bug reference: 15038
Logged by: Dominic
Email address: d(dot)huelsen(at)awo-dortmund(dot)de
PostgreSQL version: Unsupported/Unknown
Operating system: Windows Server 2016
Description:

Hi folks,

I need to upgrade an old Atlassian Confluence system which is using a
postgreSQL DB (8.4.22).
Since the newer version doesn't support v8.4.22 anymore, I need to upgrade
the database to 9.2.

I setup a test environment with the same DB version and got it running. If I
try to upgrade using pg_upgrade I experience an error, pointing towards
different lc_collate values.

In 8.4 it's "German, Germany" while in 9.2 the value says
"German_Germany.1252".
The bug is also described in BUG #14734.

I couldn't find any fixes for this.

I cannot create a 8.4 DB with "German_Germany.1252" and neither a 9.2 DB
with "German, Germany".

Anyone got a clue how to fix this?

Cheers!