CrossTextToSpeech.Current.Speak with delay

Question

i try to create Android App for my kinder to read list from String with pause between every string

i use Xamarin with xamarin.texttospeech.plugin with C#

    List<String> listword = new List<string>();

    private void AddBT_Clicked(object sender, EventArgs e)
    {
        listword.Add(MainEntry.Text);
        MainEntry.Text = "";
    }

    private void TestBT_Clicked(object sender, EventArgs e)
    {
        for (int i = 0; i < listword.Count; i++)
        {
       CrossTextToSpeech.Current.Speak(text: listword[i], crossLocale: null, pitch: (float)1, speakRate: (float)0.5, volume: 1);
          thread.sleep(30000);
        }

    }

the Problem is the App wait until the last word in the list and then speak all of them

what i have to do .