Gutenberg: Display sidebar area in the same language as the site

[AtrumGeost]

Steps to reproduce

1. Activate Gutenberg in Calypso
2. Be sure that the Calypso and wp-admin languages are set to a non-English language like Spanish
3. Add a new post using Calypso

What I expected

To see everything in Spanish (or my selected non-English language)

What happened instead

Only the content (left panel) is in the selected language. But the right sidebar is in English:

Browser / OS version

macOS Mojave 10.14.1 - Chrome Version 70.0.3538.102 (Official Build) (64-bit)

Screenshot / Video

Context / Source

User report: 8053969-hc

Was able to replicate that on a test site

[mlaetitia]

Got a user in chat reporting this issue in Swedish language: hc-8053240

Tested on a simple site (premium) in Portuguese (pt-pt) and was also able to replicate the issue (http://cld.wthms.co/y56PZf) .

[mrfoxtalbot]

I got a user in chat today reporting this same issue.

[dcoleonline]

Also reported in #1577485-zen

[gwwar]

cc @Automattic/i18n for a look if we're behind on translations or this needs more framework help

For @Automattic/lannister folks investigating, a core fetch api middleware may have broken again.

[cecoates]

Got another one. User actually didn't mind the new editor, but struggled with it being in English.

2408837-hc

[correliebre]

Also reported today in chat: #8202694-hc
And replicated on my own site.

[akirk]

I had a quick look and the problem comes from the fact that we don't have a gutenberg textdomain on translate.wordpress.com, hence no translations are provided from the PHP.

Still trying to think of the best solution here. Either:

• create a dedicated project on translate.wordpress.com for Gutenberg strings and deploy them through our i18n-deploy process.
• use the WordPress.org language packs and check them into wpcom-i18n.

We then need to load either of them on the php side.

Other opinions, maybe @yoavf?

[liviopv]

another in #7991740-hc

@akirk even though the strings don't have their own project, we can see some Gutenberg-related strings and translate them (@eduardozulian and I have made the Guten-strings 100% translated to pt-br this week), but they simply don't get deployed.

[tijosh]

Another in #1590036-zen. (Italian)

[samiff]

User reported in 4674539-hc (for Deutsch)

[serabi]

User reported in 7871537-hc (for Russian)

[gwwar]

This should be fixed by D21305-code and D21408-code, written by @mmtr and @akirk ✨

[sdixon194]

Just had a report of a user in Russian still seeing English text in the sidebar: http://cld.wthms.co/hfWGbM

1648366-zen

Would someone mind taking another peek?

[kriskarkoski]

@sdixon194 It looks like this is an Atomic site, which has a separate issue opened here: #29309

Closing this one.