using System;
using System.Windows.Forms;
using System.ComponentModel;
using System.Globalization;
using System.Drawing;
using System.Threading;

namespace Lokalisierung_ZurLauf
{
  public partial class Form1 : Form
  {
    public Form1()
    {
      InitializeComponent();
    }

    private void btnSprache_Click(object sender,EventArgs e)
    {
      if (Thread.CurrentThread.CurrentCulture.Name == "en-US") 
        SwitchLanguage("");
      else SwitchLanguage("en-US");
      MessageBox.Show(DateTime.Now.ToLongDateString());
    }

    private void SwitchLanguage(string culture)
    {
      CultureInfo cInfo = new CultureInfo(culture);
      ComponentResourceManager resManager = new ComponentResourceManager(this.GetType());
      Point old_location = this.Location;
      resManager.ApplyResources(this,"$this",cInfo);
      this.Location = old_location; //nur für .NET 1.1 nötig

      //Controls.Clear(); // <<=== Sauberer, aber ggf. mehr Flackern (es sind dann keine Apply* Methoden mehr nötig)
      ApplyRessourcesAllControls(this,resManager,cInfo); // <<=== wenn "Controls.Clear" aktiviert, dann auskommentieren.

      Thread.CurrentThread.CurrentCulture = new CultureInfo(culture);
      Thread.CurrentThread.CurrentUICulture = new CultureInfo(culture);

      //InitializeComponent(); // <<=== wenn "Controls.Clear" aktiviert, dann hier auch.
      Form1_Load(this, EventArgs.Empty);
    }

    private void ApplyRessourcesAllControls(Control control,
      ComponentResourceManager resManager,CultureInfo cInfo)
    {
      foreach (Control ctl in ((Control)control).Controls)
      {
        if (ctl.Controls.Count > 0) ApplyRessourcesAllControls(ctl,resManager,cInfo);
        resManager.ApplyResources(ctl,ctl.Name,cInfo); // folgendes nur für .NET 2.0
        if (ctl is ToolStrip) ApplyRessourcesAllToolStrips((ToolStrip)ctl,resManager,cInfo);
      }
      foreach (Component cmp in this.components.Components) // z.B. ContextMenu
        if (cmp is ToolStrip) 
          ApplyRessourcesAllToolStrips((ToolStrip)cmp, resManager, cInfo);
    }

    private void ApplyRessourcesAllToolStrips(ToolStrip ts,ComponentResourceManager resManager,CultureInfo cInfo)
    {
      foreach (ToolStripItem tsi in ts.Items)
      {
        ToolStripDropDownItem tdi = tsi as ToolStripDropDownItem;
        if (tdi != null) ApplyAllToolStripItems(tdi,resManager,cInfo);
        ToolStripComboBox tdc = tsi as ToolStripComboBox;
        if (tdc != null) ApplyAllToolStripItems(tdc, resManager, cInfo);
        resManager.ApplyResources(tsi, tsi.Name, cInfo);
      }
      resManager.ApplyResources(ts,ts.Name,cInfo);
    }

    private void ApplyAllToolStripItems(ToolStripItem tsi,ComponentResourceManager resManager,CultureInfo cInfo)
    {
      ToolStripDropDownItem tdi = tsi as ToolStripDropDownItem;
      if (tdi != null)
      {
        foreach (ToolStripItem tsi2 in tdi.DropDownItems)
        {
          ToolStripDropDownItem tdi2 = tsi2 as ToolStripDropDownItem;
          if (tdi2 != null && tdi2.DropDownItems.Count > 0) 
            ApplyAllToolStripItems(tdi2,resManager,cInfo);
          resManager.ApplyResources(tsi2,tsi2.Name,cInfo);
        }
      }
      ToolStripComboBox tdc = tsi as ToolStripComboBox;
      if (tdc != null)
        for (int i = 0; i < tdc.Items.Count; i++)
        {
          tdc.Items[i] = resManager.GetString(tdc.Name+".Items"+
            ((i == 0) ? "" : i.ToString()), cInfo);
        }
      resManager.ApplyResources(tsi, tsi.Name, cInfo);
    }

    private void Form1_Load(object sender, EventArgs e)
    {
      // Beispiel für manuellen Lokalisierungs-Aufruf
      lblBegrüssung.Text = Properties.Resources.Begrüssung;
    }

  }
}
Möglichkeit aus Bernd Marquardts Webcast (Minute 50:00):

private void SetLanguage(string strLang)
{
  Point pt = this.Location; Size sz = this.Size;
  Thread.CurrentThread.CurrentUICulture = new CultureInfo(strLang);
  this.Controls.Clear(); 
  
    this.Events.Dispose(); //dadurch würde die App nicht sauber schliessen.
  
  InitializeComponent(); 
  this.Size = sz; this.Location = pt;
}

private void btnSprache_Click(object sender, EventArgs e)
{
  SetLanguage("en-US");
  MessageBox.Show(DateTime.Now.ToLongDateString());
}

private void SetLanguage(string language)
{
  Point pt = this.Location; Size sz = this.Size;
  Thread.CurrentThread.CurrentUICulture = new CultureInfo(language);
  Thread.CurrentThread.CurrentCulture = new CultureInfo(language);
  this.Controls.Clear();
  RemoveAllHandlersExcept(this.Events, "OnMainFormDestroy");
  InitializeComponent();
  this.Size = sz; this.Location = pt;
}

const BindingFlags bf = BindingFlags.Instance | BindingFlags.NonPublic;

void RemoveAllHandlersExcept(EventHandlerList ehl, string handlerName)
{
  object head = FieldValue(ehl, "head");
  object next = FieldValue(head, "next");
  if (next != null) RemoveAllExcept(ehl, next, handlerName);
}

private void RemoveAllExcept(EventHandlerList ehl,
  object listEntry, string handlerName)
{
  object next = FieldValue(listEntry, "next");
  if (next != null) RemoveAllExcept(ehl, next, handlerName);
  Delegate handler = (Delegate)FieldValue(listEntry, "handler");
  if (handler.Method.Name == handlerName) return;
  object key = FieldValue(listEntry, "key");
  ehl.RemoveHandler(key, handler);
}

FieldInfo fi;
object FieldValue(object source, string fieldName)
{
  fi = source.GetType().GetField(fieldName, bf);
  return fi.GetValue(source);
}