Add/update newsletter messages in translation file

[arnoudhgz]

Original PR: #18932 (was closed by me because of out-of-sync fork)

Description (*)

The messages in the Newsletter module were update, but the messages in the translation files have been forgotten.

When using translation tools it is mandatory to have correct strings in the i18n files. This pull request aims to solve this.

Manual testing scenarios (*)

Compare the messages in app/code/Magento/Newsletter/Controller/Manage/Save.php with the i18n file

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)

[magento-engcom-team]

Hi @arnoudhgz. Thank you for your contribution
Here is some useful tips how you can test your changes using Magento test environment.
Add the comment under your pull request to deploy test or vanilla Magento instance:

• @magento-engcom-team give me test instance - deploy test instance based on PR changes
• @magento-engcom-team give me $VERSION instance - deploy vanilla Magento instance

For more details, please, review the Magento Contributor Assistant documentation

[arnoudhgz]

@orlangur I moved the PR to the 2.3-develop branch 👍

[magento-engcom-team]

Hi @orlangur, thank you for the review.
ENGCOM-3377 has been created to process this Pull Request

[magento-engcom-team]

Hi @arnoudhgz. Thank you for your contribution.
We will aim to release these changes as part of 2.3.1.
Please check the release notes for final confirmation.