Inconsistent accent folding with Opensearch in Wagtail admin

[mgax]

Issue Summary

I have a page with a title that contains accented characters.

With Opensearch, searching the accented word in Wagtail admin returns no results; searching for the non-accented version returns the page.

Interestingly, with the database search, the reverse is true: searching for the accented word returns the page; searching for the non-accented word returns nothing.

Steps to Reproduce

1. Run Opensearch in the background:

   docker run --rm -p 127.0.0.1:9200:9200 -e DISABLE_SECURITY_PLUGIN=true -e OPENSEARCH_INITIAL_ADMIN_PASSWORD=admin -e discovery.type=single-node opensearchproject/opensearch:2.17.1

2. Set up bakerydemo revision fade9b1
3. Install the Elasticsearch library: pip install elasticsearch==7.13.4
4. Create bakerydemo/settings/local.py with the following content:

   WAGTAILSEARCH_BACKENDS = {
       "default": {
           "BACKEND": "wagtail.search.backends.elasticsearch7",
           "URLS": ["http://localhost:9200"],
       }
   }

5. Edit the homepage and select "Translate" from the "..." menu next to the title. Select "German", set "Willkommen bei der Wagtail Bäckerei!" as the title, publish.
6. Search for Bäckerei (no resuls) and for Backerei (one result).

• I have confirmed that this issue can be reproduced as described on a fresh Wagtail project: yes

Technical details

• Python version: 3.12.9.
• Django version: 5.1.7.
• Wagtail version: 6.4.1.
• Browser version: Safari 18.3.1.

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