Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: April 29, 2019.
Scheduled code freeze: April 22, 2019

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This PR contains many translatable strings with variables that don't have a translators comment explaining them. Could we use PHPCS on Jetpack to remind people to add those comments for placeholders?

  122 | WARNING | [ ] A gettext call containing placeholders was found, but was not accompanied by a "translators:" comment on the line above to clarify
      |         |     the meaning of the placeholders. (WordPress.WP.I18n.MissingTranslatorsComment)

Could we use PHPCS on Jetpack to remind people to add those comments for placeholders?

We have started adding more and more files to a pre-commit hook that won't let you commit changes if there are phpcs warnings in a file:
https://github.com/Automattic/jetpack/blob/master/bin/phpcs-whitelist.js

Unfortunately, we can't just enable it for all files at once or it would make it very difficult to commit changes to most files in Jetpack today, so that's something that will take time.

Until then, we recommend that everyone install those tools in their editor directly (see our docs here), but that's sometimes not enough, as you've found out.

@enejb Do you think you could fix this in a new PR once #12081 is merged?

There are tools to limit the scope so that you are not blamed for pre-phpcs code: https://github.com/xwp/wp-dev-lib/blob/master/readme.md#limiting-scope-of-checks

Yes indeed. We've been exploring some options in #11545 (and previously #11515).

@akirk @jeherve here's a follow up PR to fix those errors, among some other i18n-related ones: #12332