i18n: missing translations in many parts of the admin interface

[jeherve]

Impacted plugin

Jetpack

Quick summary

Many parts of the Jetpack admin interface do not display translations, even for languages that are fully translated

Steps to reproduce

1. Install and activate Jetpack on a site running WordPress in English.
2. Go to Settings > General
3. Switch the site language to Russian, a language that is 100% translated right now
4. Go to Dashboard > Updates, and click the button to update translations
5. Ensure that there are no pending translation updates left:

6. Go to Jetpack > Settings or Jetpack > My Jetpack.
   => Note that brand names are not translated, as expected

7. On the Jetpack > My Jetpack page, only some of the strings are translated

8. On the Jetpack > Settings page, only some of the strings are translated

Site owner impact

Between 20% and 60% of the total website/platform users

Severity

Major

What other impact(s) does this issue have?

No revenue impact

If a workaround is available, please outline it here.

No response

Platform (Simple and/or Atomic)

Self-hosted, Atomic

[jeherve]

Noting that this was brought up in this Slack thread:
https://wordpress.slack.com/archives/C02RP50LK/p1782253517021789

In there, we discussed other issues with translations, which may be caused by language pack generation issues.

[jeherve]

Related: I18N-371