Extra query to get WPLANG option on each page-load when the option doesn't exist in the db

[aristath]

On each page-load we call determine_locale() which in turn tries to get the WPLANG option using get_option.
When the site language is en_US and has never been changed, the WPLANG option doesn't exist in the options table in the database.
As a result, get_option doesn’t find it autoloaded and then triggers a query.

If the language is changed to something else and then back to en_US, the option is saved as an empty string and gets autoloaded.

Proposed solution:
The simplest solution that comes to mind (which also wouldn't interfere with the way multilingual plugins work) would be to check if the row exists in the table, and if not just create it and save as empty string. It can then get picked-up by the object-caches and spare us the extra query.

[SergeyBiryukov]

This can be done in an upgrade routine like upgrade_550()​, for example.

Adds a default value for WPLANG on new installations and introduces a new upgrade routine for WP 5.6.0.

Trac ticket: https://core.trac.wordpress.org/ticket/50980

[aristath]

Just a note here: I noticed that all upgrade processes mention database-versions and that's how things get triggered.
I don't know how these get generated or what the db-version will be in order to add that in the patch, is there documentation somewhere about that?

[audrasjb]

Just added a comment in the PR about a very small WPCS point

The current test failure is valid and needs to be investigated:

1) Tests_L10n_GetLocale::test_network_option_should_be_fallback_on_multisite
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'es_ES'
+'en_US'
/var/www/tests/phpunit/tests/l10n/getLocale.php:54

A similar upgrade routine was done in 4.0:
​https://github.com/WordPress/wordpress-develop/blob/c7f5af72dcc4f36365656fdbfccfa0fb51fefd1f/src/wp-admin/includes/upgrade.php#L1921-L1933

Failing tests were fixed.
Tested on single-site and multisite installations :+1:

[SergeyBiryukov]

It looks like the tests are still failing with the latest patch, so this needs some more work:
​https://travis-ci.com/github/WordPress/wordpress-develop/jobs/386888985

With 5.6 RC in a few days, moving to the next release for now.

[lukecarbis]

With not much activity happening here and beta 3 now out for 5.7, I'm moving this to 5.8.

[JeffPaul]

Given no traction on resolving the failing tests since the 5.6 release cycle I'm going to punt this out of the 5.8 milestone to Future Release. If someone is able to update the linked PR to resolve those failures, then this ticket would be good to get added back to a numbered milestone.