**Comments**  

Comments should describe **why** something is done in the way it is done. Describing **what** is done does not add any value.  

Comments like  

>     // Create form and its fields
>     // Translations
>     // Design form and its fields
>     .....

are also a sign that this parts maybe should be extracted to separate methods.  

You can find a very good read about comments here: [https://codereview.stackexchange.com/a/90113/29371][1]  


**Naming**  

Classes should be named using nouns or noun phrases ([naming guideline][2]), so a name like `Validate` is better suited for a method. You should rename the class to `Validator` or better `ProductPriceValidator` to make it more clear what the class is about.  
In addition the name `Prompt` does not reflect what the class is doing. A better name would be `ProductPriceView`.  

**ShowPriceDialog**  

This method is in its current state clearly too long. Also, why did you make the class and all of its methods static ?  

I would make the class and its methods `non static` and would add methods like  

- `InitializeComponent()` for creating the view  
- `ResetView()` for setting the default values of the form  
- and keep the `ShowPriceDialog()` method, but I would change the returned value from `decimal` to `DialogResult` and adding a `Prize` property to get the entered prize. This needs a change in the textbox eventhandlers to call `Hide()` on the view rather than `Close()`.   

___
Instead of using  

>     if (this.textBoxAddProduct.Text.Trim().Length == 0)  

you should use [`string.IsNullOrWhiteSpace()`][3] which is checking wether the passed in string is `null` or does only contain white space characters.  

___  

Assigning `string.Empty` makes the intention more clear than assigning `""` which by the way with older eyes not always is easy to distinguish at the first glance from `" "`.  

___  

Because you aren't changing these fields  

>     private string defaultTextAddProductTextBox;
>     private string messageMissingProductname;

you should make them `readonly`.



**Validation**  

IMHO the `IsValidPriceFormat()` has a decent flaw. You are assuming that the entered price will have a comma as decimal separator. If you plan to localize it to make it usable for e.g an american user, you would not only need to change the regex, but to provide a different validator so you won't break the german users.  

A much better way to validate the passed price would be to use [`decimal.TryParse()`][4] which returns a boolean indicating if the given string could be parsed to a deciaml value. At the provided link location you can find a example of how to use this.  

A useful component could also be a [`MaskedTextBox`][5]


  [1]: https://codereview.stackexchange.com/a/90113/29371
  [2]: http://msdn.microsoft.com/en-us/library/ms229002.aspx
  [3]: https://msdn.microsoft.com/en-us/library/system.string.isnullorwhitespace%28v=vs.110%29.aspx
  [4]: https://msdn.microsoft.com/en-us/library/ew0seb73%28v=vs.110%29.aspx
  [5]: https://msdn.microsoft.com/en-us/library/system.windows.forms.maskedtextbox%28v=vs.110%29.aspx