Add "optional" translation in checkout password field

[Jeroenll]

Description

Original PR #10056.

Translatable placeholder is added in the template but not the .csv file.

Fixed Issues (if relevant)

1. None that I could find.

Manual testing scenarios

1. Install non English language pack.
2. Open Guest Mode browsing.
3. Go to checkout.
4. Enter the email address of a customer which already exists.
5. Placeholder is not translated.

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)