Wpf localization affect date format only at runtime - FrameworkElement

Question

I have a WPF applciation, one of functionality is convert the date acoording to the CultureInfo.

the cultureinfo.Culture by defalult in my applciation is "en-GB".

And I use the element FrameworkElement : so the "en-US" is the default cultureinfo.Culture.

I change it at the runtime with this line in my code:

FrameworkElement.LanguageProperty.OverrideMetadata(typeof(FrameworkElement), new FrameworkPropertyMetadata(XmlLanguage.GetLanguage(CultureInfo.CurrentCulture.Name)));
       

In this case , the cultureinfo.Culture in the FrameworkElement is changed successuffly to "en-GB"

and I test in debugging (in the immediate command in visual studio) :

DateTime date;
         if (DateTime.TryParse("05/27/2021", out date);

and the result is FALSE! ( it is OK) Also :

DateTime date;
         if (DateTime.TryParse("30/05/2021", out date);

and the result is TRUE! ( it is OK)

But the problem in fact I get the date from an external excel file, and I get the date from this file without any problem, but unfortunately always I have the result for this : "not a valid date" that is to say it takes the "en-US" format!!

I don't understand how it make this?!

this is the essential part of my code:

public override DataTemplate SelectTemplate(object item,
                                            DependencyObject container)
    {
        DataTemplate template = null;
        if (item != null)
        {
            var objectType = (item as IEnumerable<object>).FirstOrDefault();
            if (objectType != null)
            {
                FrameworkElement element = container as FrameworkElement;

and the object item contains the list of dates from my excel file.

Thanks