Installing a module causes translations to be overwritten

Problem/Motivation

I was having a weird problem in a WebTestBase test that I am using to generate screenshots in Hungarian for the translation of the User Guide, and I traced it down to a weird bug that is either a problem in Drupal Core or localize.drupal.org, or some combination.

I was able to reproduce this bug on simplytest.me with the following steps:

a) Install Drupal Core 8.2.0-rc2 in English, with the Standard profile. Make sure that you check the "Check for updates" box, so that the Update Manager module is installed.

Note: In comment #17, this was retested in Drupal 8.6.x as of April 30, 2018, and it is still a bug.

b) Install the 4 multilingual modules from admin/modules.

c) Go to admin/config/regional/language and add Hungarian. Translations are imported from localize.drupal.org. Hungarian is 99% translated today in RC2 -- there are only 2 untranslated strings.

d) Set Hungarian as default language and delete English. (Note: this is interesting to try to do when you do not read Hungarian! Quite an adventure. Luckily, I am "slightly familiar" with the Drupal admin UI and URLs. :) ) At this point, you can verify that the Toolbar across the top of the screen is fully in Hungarian. In particular, the left-most link from the Manage menu (Content in English) says "Tartalom".

e) Go to admin/modules and install the core Activity Tracker module. Translations are imported. Note: I have no idea why translations would be imported at this time, since weren't they already imported for Core? But they are definitely being imported... I can see this in my test as well.

f) Go to admin/config/regional/translate and search for the word "Content". You will see that its translation has become "Content" (English) instead of "Tartalom". Content type is also screwed up, but some other words are still translated... Screenshot:

In my tests, after this point the Toolbar in screenshots is sprinkled with English words amongst the Hungarian. The words "Help" and "People" and "Content" are in English, while the rest is still in Hungarian. But the root cause appears to be that some translations are being overwritten.

I have no idea what the root cause of this happening is... and I think I have a way to work around it for my screenshots (on admin/config/regional/translate/settings I should be able to set it so imports do not overwrite existing translations. Hopefully.) Also note that in my tests I have verified just before installing this module that the word Content is translated to Tartalom and just after it is Content, so I am quite sure it is the translation import from this module install that is causing the problem.

Proposed resolution

1. Figure out why:

a) Translations are being imported when installing a Core module even though Core translations have already been imported when I added the language.

b) When they are imported, they are overwriting some words with English instead of Hungarian.

The problem occurs because the locale_modules_installed() sets the langcode of installed config before the installed config has been translated. This means that the \Drupal\locale\LocaleConfigSubscriber creates translations into the site's default language using the english in the content.

2. Fix the problem(s) by not setting the config's langcode in locale_modules_installed(). Set it as part of translating the configuration in the locale batch that runs after configuration has installed. This results in the system not saving incorrect translations and configuration being correctly translated.

Remaining tasks

Make a patch with a test.

User interface changes

UI translations will not be overwritten when installing a Core module.

API changes

• Add new method LocaleConfigManager::updateDefaultConfigLangcodes()
• Add new argument $update_default_config_langcodes to locale_config_batch_build() and locale_config_batch_update_components()
• New batch callback locale_config_batch_set_config_langcodes()

Data model changes

None