change the UI language at run time at user selection in windows application in c#.

set CurrentCulture and CurrentUICulture based on user selection

// Put the using statements at the beginning of the code module
using System.Threading;
using System.Globalization;
// Put the following code before InitializeComponent()
// Sets the culture to French (France)
Thread.CurrentThread.CurrentCulture = new CultureInfo("fr-FR");
// Sets the UI culture to French (France)
Thread.CurrentThread.CurrentUICulture = new CultureInfo("fr-FR");

ref :http://msdn.microsoft.com/en-us/library/b28bx3bh(v=vs.110).aspx[^]

If you need to change the culture on button click or menu item selection etc.. check below article
Changing Your Application User Interface Culture On The Fly[^]
And this SO answer How do I change the culture of a WinForms application at runtime[^]