How do I set CultureInfo.CurrentCulture from an App.Config file?

Starting form .Net 4.5 it's possible to set the default thread culture so there is no need to fix it per thread:

CultureInfo.DefaultThreadCurrentCulture = new CultureInfo("pt-BR");
CultureInfo.DefaultThreadCurrentUICulture = new CultureInfo("pt-BR");

I haven't yet found a configuration that matches web.config globalization section unfortunately.

I don't know a built-in way to set it from App.config, but you could just define a key in your App.config like this

<configuration>
    <appSettings>
        <add key="DefaultCulture" value="pt-BR" />
    </appSettings>
</configuration>

and in your application read that value and set the culture

CultureInfo culture = new CultureInfo(ConfigurationManager.AppSettings["DefaultCulture"]);
Thread.CurrentThread.CurrentCulture = culture;
Thread.CurrentThread.CurrentUICulture = culture;

Also, as @Ilya has mentioned, since .NET 4.5 you can set the default culture once, rather than per-thread:

CultureInfo.DefaultThreadCurrentCulture = culture
CultureInfo.DefaultThreadCurrentUICulture = culture