BUG #14170: error message in case LOCALE de_DE.UTF8 is used in combination with CLIENT_ENCODING=LATIN1

The following bug has been logged on the website:

Bug reference: 14170
Logged by: Dag Hammerich
Email address: dag-hammerich(at)gmx(dot)de
PostgreSQL version: 9.4.8
Operating system: Ubuntu 14.04 LTS
Description:

I think postgres cannot correctly "translate" its error message for this
special situation because for LOCALE "de_DE" it seems that "double
quotsation marks left" are used that are not defined in (for example)
LATIN1.

To reproduce it the following could be done:

1. create new DB-cluster by with corresponding locale:
initdb --locale=de_DE.UTF8

2. set CLIENT_ENCODING or environment variable PGCLIENTENCODING to LATIN1

3. produce any error that forces postgres to react with an error message,
e.g. by a syntax error in SQL or dropping a DB that doesn't exist using this
command line:
dropdb xyz

>>> this will result in:
ERROR: character with byte sequence 0xe2 0x80 0x9e in encoding "UTF8" has no
equivalent in encoding "LATIN1"

The mentioned byte sequence are „German double quots left”.