Windows 8.1 Multilanguage OS and Store language

Question

Hi everyone, I'll try to explain my problem the best I can:
I am trying to deploy a Windows 8.1 image using ConfigMgr 2012 R2. The image is configured as a multilingual image using French (Default) and English.
My deployment is working fine and I am using the Unattend.xml file to set the language settings to either FR-CA, FR-FR (UILanguage).
My problems are the following:
Modern App are showing in English.
Windows Store is displayed in English.
I did found that if you go to Control Panel\Clock, Language and Region\Language there is bot English and French language displayed there.
The language listed at the top will define the language the Windows Store will use when installing Apps and the language displayed. So if French is at the top, the store will be in French.
My problem is that even if I am setting all the languages in the Unattend.xml to either FR-FR or FR-CA, the store will remain in English unless I manually place the French at the top in the control panel.
There is the HKCY\Control Panel\Desktop\PreferredUILanguages key that will also define that setting.
To make this automated, there is the GPO called Restricts the UI Languages Windows should use for the selected user that I can use, the problem with this setting is that it will restrict the language with no possible way to revert to English which will ruin
my effort for a Multilanguage image that the user can decide.
Anyway, my question is, I am missing something about how to set the language? Is there any other setting that I can use in the unattended.xml to set the PreferredUILanguages key or do I have to use either two different policies or set it by using GPP?
Thank you all for your help!
Mathieu
Edit: I tried using the GPP setting with no success. The way I am looking at this behavior seem a bit buggy. When manually selecting stuff, the setting is right but setting it programmatically, this is not.
When selecting English manually, all the user's interface is in English, when doing the same thing manually for French no problem. With a freshly deployed image, settings seems rights except the part I am talking about.

Answer 1

Like I said I did tried the XML on a deployed computer I am using this command Control intl.cpl,, /F:"Path to file"
The file content is this one:
<gs:GlobalizationServices xmlns:gs="urn:longhornGlobalizationUnattend">
<!--User List-->
<gs:UserList>
<gs:User UserID="Current">
</gs:User>
<!--Display Language-->
<gs:MUILanguagePreferences>
<gs:MUILanguage Value="fr-FR">
<gs:MUIFallback Value="en-US">
</gs:MUIFallback>
</gs:MUILanguage>
</gs:MUILanguagePreferences>
</gs:UserList>
</gs:GlobalizationServices>
On a live computer I do not get any result and there is nothing in the event viewer neither.
Is there a way you could test or validated with my file?
Mathieu