Question: How to adjust text Yes/No of WPF Confirmation Message Box buttons for other culture?

Is it possible?

In fact I need to show: Si/No

Answer: 
You cannot localize message box buttons. They follow the OS's language. There may be some Win32 API hacks but I have no experience about those.
You could create a style to your buttons and use a converter to do the translation from english to spanish. In the example below the titleTranslator converter is used to translate this button's text.

```
<Style x:Key="myStyle" TargetType="{x:Type Button}">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type Button}">
                <Border CornerRadius="6 6 0 0" VerticalAlignment="Center" Margin="5">
                    <TextBlock Text="{Binding Converter={StaticResource titleTranslator}}" FontSize="44" FontWeight="Bold" Foreground="{Binding Foreground, Converter={StaticResource brushConverter}}" />
                </Border>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

```