And finally, I found the bug. I replaced culture with tr_TR and done.

public async Task<string> MediaToolKitSpeechToTextAsync(){

var options = new SpeechToTextOptions
{
    Culture = CultureInfo.GetCultureInfo("tr_TR") 
};

await SpeechToText.Default.StartListenAsync(options, CancellationToken.None);

SpeechToText.Default.RecognitionResultCompleted += (s, args) =>
{
    if (args.RecognitionResult != null && !string.IsNullOrEmpty(args.RecognitionResult.Text))
    {
        transcript = args.RecognitionResult.Text;
    }
    else
    {
        MainThread.BeginInvokeOnMainThread(async () =>
        {
            await DisplayAlert("Hata", "Ses anlaşılamadı.", "Tamam");
        });
    }
};

return transcript;

}