Checklist

• I have updated to the latest available Home Assistant version.
• I have cleared the cache of my browser.
• I have tried a different browser to see if it is related to my browser.
• I have tried reproducing the issue in safe mode to rule out problems with unsupported custom resources.

Describe the issue you are experiencing

My HA is set to use German, and the country is set to Switzerland. The system's locale settings are German/Switzerland, too. And there's no issue with formatting in general. I set the user to use "system settings" for the formatting.

Yet numbers are formatted using German rather than Swiss rules in HA: our decimal separater is a period, not the comma ("0.5" rather than "0,5"). German uses the dot as a thousand separator instead. This is particularly irritating with larger numbers using three decimals (eg. Total Energy 45,923kWh). I'm constantly confused wondering whether that means we really used 45 _giga_Wh. Or there's 1.900W instead of 1900W (or 1.9kW).

The following screenshot shows HA on the left, the Mac's settings on the right:

As can be seen from the screenshot the Swiss formatting isn't even available as an option.

I'm seeing the same on Mac, web UI and iOS app.

Describe the behavior you expected

The formatting for Switzerland should be 12'345.67. See eg. https://www.localeplanet.com/icu/de-CH/index.html

Steps to reproduce the issue

1. Configure your operating system for German/Switzerland
2. Configure your HA the same, and the user to use the system settings
3. Check out some decimal or large numbers

What version of Home Assistant Core has the issue?

2025.9.4

What was the last working version of Home Assistant Core?

No response

In which browser are you experiencing the issue?

Safari 26.0.1 (21622.1.22.11.15) (and others...)

Which operating system are you using to run this browser?

macOS Tahoe 26.0.1, iOS 26.0.1

State of relevant entities

sensor.athom_smart_plug_waschkuche_energy

45.9419212341309

state_class: total_increasing
unit_of_measurement: kWh
device_class: energy
icon: mdi:lightning-bolt
friendly_name: Smart Plug Waschküche Total Energy

Problem-relevant frontend configuration

JavaScript errors shown in your browser console/inspector

Additional information

No response