I've been having an issue with locale overriding in my Outlook Add-in. I recently added some simple override logic to the manifest to point the user to our UK sign in process if they are in a UK locale. For example:
<SourceLocation DefaultValue="https://our-url.com/assets/outlook.html">
<Override Locale="en-gb" Value="https://our-url.co.uk/assets/outlook.html" />
</SourceLocation>
This works correctly in outlook web, so the overrides appear to be implemented correctly. All the urls are the UK versions when I change my language to English(United Kingdom). However it does not work in Outlook desktop. It's picking up the locale as en-us regardless of any settings I change in Outlook or on my PC. We've also had an internal user located in the UK attempt to use the add-in and they still see the US version in Outlook desktop.
Edit: I'm on Windows 7 with Outlook 2016 (Also tested on Windows 10 with Outlook 2016). I've tried setting the editing language under Outlook's options menu to English (United Kingdom) as well as changing the date format, location, key board language, and system locale under the Windows control panel to English (United Kingdom) and changing the Outlook display language settings to match Microsoft Windows. There doesn't seem to be any variants of the english display language pack for Outlook. Just one general english pack.
What setting specifically is checked by Outlook desktop to determine locale?