Improve I18N Issues (Based on 1.17)

[alexclassroom]

Summary

This PR can be summarized in the following changelog entry:

• Improves translation handling for post types registration and indexation reset. Props to @alexclassroom.

Relevant technical choices:

Milestone

• I've attached the next release's milestone to this pull request.

Test instructions

Test instructions for the acceptance test before the PR gets merged

This PR can be acceptance tested by following these steps:

Basically a regression test since strings may not be there (and definitely the second one isn't since it's new)

• Enable the custom post types (Books and Movies)
• check that you still see "Add new" in their submenus
• reset indexables and migrations
• visit the Yoast SEO General page and see that you get a notification saying Because some of your SEO data was reset by the Yoast Test Helper, your SEO data needs to be reprocessed.

Test instructions for QA when the code is in the RC

• QA should use the same steps as above.

QA can test this PR by following these steps:

Fixes #

[enricobattocchi]

Thanks @alexclassroom! We're adding this to the upcoming 1.18 release.