[VS2015]: Error with designer when Localizing an Application

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Visual-Studio

XAML

WPF

Hi everybody,

I have Visual Studio 2015 and I've just followed this[^] tutorial in order to learn how to localize a WPF application correctly. Everything works fantastically, but in Visual Studio I get a strange error. Here's a screenshot.
Despite that, everything compiles well and Localization works, but it's quite annoying.

I googled for it and I found out this:
The string values used for Name have some restrictions, as imposed by the underlying x:Name Directive defined by the XAML specification. Most notably, a Name must start with a letter or the underscore character (_), and must contain only letters, digits, or underscores.

But it's for names, the property which gives me the problem is a Title.

Does anybody know how can I get rid of it?

Thanks a lot!

LG

EDIT: In the screenshot I've translated the error into English. That's why you clearly see that the image has been edited. I absolutely do not want to post "fake questions". :)