Date separators does not change format when date style is changed

**Describe the bug**
Options in date separators does not change format to D/M/YYYY when date style is changed to "7 January 2018" but it does when date style is changed to "7/1/2018"

**To Reproduce**
Steps to reproduce the behavior:
1. Go to 'Admin'
2. Click on 'Formatting'
3. Change Date style to '7 January 2018'
4. See Date separators

**Expected behavior**
Date separators changes from 'M/D/YYYY' to 'D/M/YYYY' just like it does when date style is set to '7/1/2018'

**Screenshots**
Where the date separator change does work

Where the change doesn't work

**Information about your Metabase Installation:**

You can get this information by going to Admin -> Troubleshooting.

- Your browser and the version: Firefox 77.0
- Your operating system: Windows 10
- Your databases: Postgres
- Metabase version: v0.35.4
- Metabase hosting environment: Docker
- Metabase internal database: Postgres
- Metabase Report Timezone: Asia/Calcutta

Start in Admin > Formatting and reproduce the issue by changing Date style to “7 January 2018,” then inspect the date separator behavior. Done means the separator changes from M/D/YYYY to D/M/YYYY for that style, matching the behavior of “7/1/2018”; the issue names no source file or test to run.

Written by the indexing model from the issue text.