看起来像是在用事件处理程序内的这行代码重置窗体，该事件处理程序在窗体的选定索引更改时触发

this.Controls.Clear();

尝试删除或注解掉该行，并尝试更改组合框项，使新方法看起来与下面的方法类似

private void ComboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            switch (ComboBox1.SelectedIndex)
            {
                case 0:
                    Thread.CurrentThread.CurrentUICulture = new System.Globalization.CultureInfo("en-US");
                    break;
                case 1:
                    Thread.CurrentThread.CurrentUICulture = new System.Globalization.CultureInfo("de-DE");
                    break;
                    
         }
}

如果在窗体加载时组合框为空，则可以定义一个填充组合框的方法，并从FormLoaded事件处理程序调用该方法。

//inside the constructor of your form, register the event handler that is triggered when the form is loaded
public MyForm{

Load += OnLoad;
}
//then in the Load event handler fill the combo box
private void OnLoad(object? sender, EventArgs e)
{
      FillComboBox();
}
void FillComboBox(){
   var items = new string[]{"English","French","Chinese","Hindi"};
   foreach(var item in items){
    //assuming you have a definition for the combo box
    comboBox.Items.Add(item);
   }
}