How to make ‘input type=”date”‘ display as ISO-8601 in Firefox?

Here is the date picker I am testing with.

According to its documentation,

[t]he appearance of the date picker input UI varies based on the browser and operating system.

The date is displayed as DD/MM/YYYY.

Expected behaviour: The date is displayed as YYYY-MM-DD.

1. How do I make it so?
2. Can I do this without touching language settings?
   – Yes, technically all legible notations are/encode languages, but this is not exactly a common interpretation or a helpful one. I want to use the “language” ISO-8601 for time and the language “British English” for text.

Firefox 150.0.1 (64-bit) on Arch Linux.

/etc/locale.conf:

LANG=en_GB.UTF-8
LC_TIME=en_DK.UTF-8

Console output of date:

$ date
2026-05-07T19:04:58 CEST

According to Wikipedia,

[t]he Government of Canada specifies the ISO 8601 format for all-numeric dates (YYYY-MM-DD; for example, 2026-05-07).

In Firefox under Settings|General…

Language was originally set to

1. English (UK)
2. English (US)

I tried setting Language to

1. English (CA)
2. English (UK)
3. English (US)

but it only saves as

1. English (CA)
2. English (US)
3. English (UK)

The option

Use your operating system settings for “English (Denmark)” to format dates, times, numbers, and measurements.

does not make a difference: It is enabled, I also tried to disable it.