This is a project that has developers from different continents, so we need English in messages and mails.

Web GUI:
manage Jenkins,
configure system,
(email sending is duly configured),
press "Test configuration by sending e-mail to System Admin Address"

Result: A test mail is received that is English. Excellent!

Now we configure a job with "Send separate e-mails to individuals who broke the build".
The build breaks.

Result expected: A mail gets sent to the people who potentially broke the build in English.
Result seen: A mail gets sent to the people who potentially broke the build in German.

I perceive the "Send separate e-mails to individuals who broke the build"
and the "Test configuration by sending e-mail to System Admin Address"
as both Jenkins core functionality. I use the "Test configuration" functionality,
well, to test the configuration. I want to be able to test the language setting as well.

This ticket can be closed if the mail that gets sent for "test configuration"
is locallized in the same way as the "build broke" mails.

This is somewhat of a sybling ticket to the feature request
https://issues.jenkins-ci.org/browse/JENKINS-9668
"Language control for "build broke" mails."