If you're creating a site where you are allowing the user to change language for example, then you need to perform this in the Global.asax
file in the Application_BeginRequest
method.
Each request will then have the culture set.
You simply set the following 2 lines:
Threading.Thread.CurrentThread.CurrentCulture = new System.Globalization.CultureInfo("fr-FR");
Threading.Thread.CurrentThread.CurrentUICulture = new System.Globalization.CultureInfo("fr-FR");
The first line will set the number/date/etc formatting.
The second line specifies which resource localization to load - which will contain your translated content.