Kibana version: 6.5.0

Elasticsearch version: 6.5.0

Server OS version: Docker container

Browser version: Firefox, Chrome

Browser OS version: Windows

Original install method (e.g. download page, yum, from source, etc.): Docker

Describe the bug:
When I go to Advanced Settings in Management I see Formatting locale set to be-nl despite the default being en. I think this may be because the default setting of en does not appear in the list so it chooses the first.

Steps to reproduce:

1. Open Kibana, and go to Management => Advanced Settings

2. Scroll down to Formatting locale and note that it is set to be-nl
   

3. Change it to something (e.g. en-gb) and click save.

4. Now Default: en appears, plus a Reset to default button
   

5. Click Reset to default

6. Note that it resets back to be-nl.
   

Expected behavior:
Resets back to the default that it says (en), or change the default from en

Any additional context:
My locale is en-gb