question

That depends on what you mean by default language.

• The language used by the Edge UI (menus, dialogues etc.) is governed by the application locale policy, which also determines how a date, for example, is formatted when the page it's on doesn't provide a specification.
  https://docs.microsoft.com/en-us/deployedge/microsoft-edge-policies#description-177
  

• The default language to be used on websites offering more than one language (e.g. google.com) is determined by the preferred languages policy, which are listed in order of preference:
  https://docs.microsoft.com/en-us/deployedge/microsoft-edge-policies#definepreferredlanguages
  

• The language(s) used to check spelling are specified by the spellcheck languages policy:
  https://docs.microsoft.com/en-us/deployedge/microsoft-edge-policies#spellchecklanguage
  This policy depends on others as described in the documentation.
  

• The language used for the New Tab Page's content is specified on that page, using the Advanced option from the Page layout menu accessed via the Page Settings gear icon. It can't be set centrally, but it will by default use the application locale.

  
  
  
  
  

Hi @SteveArmitage-8873,

Thanks for posting in the Q&A.

The policy of DefinePreferredLanguages: If you enable this policy, websites will appear in the first language in the list that they support unless other site-specific logic is used to determine the display language. The language variants defined in this policy override the languages configured as part of the SpellcheckLanguage policy.

1.We could use group policy to change from English US to English UK:
Install administrative templates, add rules and settings for Microsoft Edge, GP unique name: DefinePreferredLanguages
GP name: Define an ordered list of preferred languages that websites should display in if the site supports the language

About configuring Microsoft Edge policy settings on Windows:
https://docs.microsoft.com/en-us/deployedge/configure-microsoft-edge

2.We could also use the reg key to change it:
Under HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Edge, create a new string value named DefinePreferredLanguages
Set the value to be en-UK

Please refer to this link:
https://www.reddit.com/r/MicrosoftEdge/comments/kht2fy/setting_the_locale_to_enau_via_gpo_does_not/
Note: Non-Microsoft link, just for the reference.

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