You can add this NuGet Package Microsoft.Windows.SDK.Contracts so you can access Windows.System.UserProfile.GlobalizationPreferences from WPF.
Then you can call this code from your WPF Application:
var languages = Windows.System.UserProfile.GlobalizationPreferences.Languages.ToList();
It will contain a list of string of languages that are preferred by the user, in order of preference. On my machine, it returns "pt-BR":

References:
https://blogs.windows.com/windowsdeveloper/2019/04/30/calling-windows-10-apis-from-a-desktop-application-just-got-easier/#eAjFgDgbqEcJZIqb.97
Note that it requires default package management format set to PackageReference, and NuGet 4.0 or higher.
You can do it on Visual Studio 2019 using the context menu:
