Fix strings in release/16.6

[loremattei]

Translators noticed that some strings which reference other strings resources have been sent to GlotPress.
I think the only issue with those is that they miss the translatable=false attribute, so this PR adds it.

Adding @zwarm and @malinajirka as reviewers to make sure this is the expected behaviour.

After this PR is merged, we'll need to merge the release branch into develop to update GlotPress.

cc @JavonDavis.

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APK here.

[zwarm]

@loremattei - Works as expected. Thanks for catching this.