There are 10 non-deleted instances that have a P218 mapping.

https://furry.wikibase.cloud/ - 1 statement
https://riga-literata.wikibase.cloud/ - 6 statements
https://data.r74n.com/ - 6 statements
https://enote-playground.wikibase.cloud - 0 statements
https://bodhitestwiki.wikibase.cloud/ - 0 statements
https://fuzzy-sl.wikibase.cloud/ - 0 statements
https://ontolagoon.wikibase.cloud - 0 statements
https://memory-prime.wikibase.cloud - 2 statements
https://recordsincontexts.wikibase.cloud - 0 statements

Example of the SPARQL query I used to find this info:

PREFIX wdt:  <https://recordsincontexts.wikibase.cloud/prop/direct/>
PREFIX p:    <https://recordsincontexts.wikibase.cloud/prop/>
PREFIX ps:   <https://recordsincontexts.wikibase.cloud/prop/statement/>

SELECT ?item ?value ?statement WHERE {
  ?item wdt:P275 ?value .
  ?item p:P275 ?statement .
  ?statement ps:P275 ?value .
}

to be discussed with tech lead before moving to team refinement (sanity check)

Reasoning for choosing the solution:

We see the following ways to proceed:

• Do nothing
  • This would leave Cloud inconsistent with Wikidata
• Update the expected property code from P218 to P305
  • The users who already mapped their property and created statements in the language Items based on it will have to remap the property and rewrite all those language statements.
  • Optionally, we can try to write a script that makes the update.
• Allow both properties to be mapped to Wikidata
  • Creates a slight inconsistency with Wikidata, but we avoid affecting the existing users.

Currently there are 9 non-deleted instances that configured this special property and mapped it to P218.

SELECT
  ws.wiki_id,
  w.domain, 
  JSON_UNQUOTE(JSON_EXTRACT(ws.value, '$.properties.P218')) AS p218_mapped_to
FROM wiki_settings ws
JOIN wikis w ON w.id = ws.wiki_id
WHERE ws.name = 'wikibaseManifestEquivEntities'
  AND ws.value LIKE '%"P218"%'
  AND w.deleted_at IS NULL;

Only 4 of them used it in a statement, with the total of 15 statements.
https://furry.wikibase.cloud/ - 1 statement
https://riga-literata.wikibase.cloud/ - 6 statements
https://data.r74n.com/ - 6 statements
https://memory-prime.wikibase.cloud - 2 statements

Based on this, we believe that we should update the code and inform the owners of 4 instances that they should update their mapping and statements.

from the daily: we'll go ahead and deploy this today and @Anton.Kokh will notify anybody who needs to be notified afterwards

To validate that this change works as intended:

• On a Wikibase Cloud Wiki enable Lexeme
• Create a property with Label IETF language tag and type string
• enter the equivalency on Wikibase cloud dashboard of this new property to P305
• Create an item with Label French
• Add a statement to the French item using IETF language tag property that it has the language tag fr
• Open the special new lexeme page
• Confirm that French is now returned when filling in the Lexeme's language field and that no language code box appears

I would like to humbly reopen this for a small investigation.
Previously, the language code property had to be an External Identifier. The properties on Wikidata are also external identifiers.
Now for some reason it works only if the property is a String.
Any idea why the behavior changed?

After investigation, moving this back to Done and opening a bug report.

Reported in T429998: Stale LexemeLanguageCodePropertyId persists in localStorage after manifest update, only clearing the ResourceLoader store fixes it