"hello".toLocaleUpperCase() and Intl.getCanonicalLocales(['und']) issues

Problem Description

"hello from 0.78".toLocaleUpperCase() returns "uppered" string not "HELLO FROM 0.78" what i was expecting and is returned in RNW 0.77.

Intl.getCanonicalLocales(['und']) fails to RangeError: Invalid language tag: und.

Same issues now also on RNW 0.79.

Steps To Reproduce

Create HelloWorld 0.78 or 0.79

npx @react-native-community/cli init hello078 --version "0.78.2"
cd hello078
yarn add react-native-windows@0.78.6
npx react-native init-windows --overwrite --template old/uwp-cs-app

add logs into app

const localeHello = 'hello world 0.78'.toLocaleUpperCase();
const hello = 'hello world 0.78'.toUpperCase();
console.log({hello, localeHello});

and log is

LOG {"hello": "HELLO WORLD 0.78", "localeHello": "uppered"}

Expected Results

LOG {"hello": "HELLO WORLD 0.78", "localeHello": "HELLO WORLD 0.78"}

CLI version

15.0.1

Environment

info Fetching system and libraries information...
System:
  OS: Windows 10 10.0.19045
  CPU: (12) x64 Intel(R) Core(TM) i9-8950HK CPU @ 2.90GHz
  Memory: 17.64 GB / 31.90 GB
Binaries:
  Node:
    version: 20.18.0
    path: C:\Program Files\nodejs\node.EXE
  Yarn:
    version: 1.22.19
    path: C:\Program Files (x86)\Yarn\bin\yarn.CMD
  npm:
    version: 10.8.2
    path: C:\Program Files\nodejs\npm.CMD
  Watchman: Not Found
SDKs:
  Android SDK: Not Found
  Windows SDK:
    AllowDevelopmentWithoutDevLicense: Enabled
    AllowAllTrustedApps: Enabled
    Versions:
      - 10.0.18362.0
      - 10.0.19041.0
      - 10.0.22000.0
      - 10.0.22621.0
      - 10.0.26100.0
IDEs:
  Android Studio: Not Found
  Visual Studio:
    - 17.14.36202.13 (Visual Studio Community 2022)
Languages:
  Java: Not Found
  Ruby:
    version: 2.6.1
    path: C:\Ruby26-x64\bin\ruby.EXE
npmPackages:
  "@react-native-community/cli":
    installed: 15.0.1
    wanted: 15.0.1
  react:
    installed: 19.0.0
    wanted: 19.0.0
  react-native:
    installed: 0.78.2
    wanted: 0.78.2
  react-native-windows:
    installed: 0.78.6
    wanted: 0.78.6
npmGlobalPackages:
  "*react-native*": Not Found
Android:
  hermesEnabled: true
  newArchEnabled: true
iOS:
  hermesEnabled: Not found
  newArchEnabled: Not found

Community Modules

No response

Target Platform Version

None

Target Device(s)

No response

Visual Studio Version

None

Build Configuration

None

Snack, code example, screenshot, or link to a repository

No response