При выборе TextBox менялась раскладка клавиатуры C#

You can change the input language programmatically using InputLanguage.CurrentInputLanguage.

It's enough to handle Enter event of your control and set the InputLanguage.CurrentInputLanguage to desired language and also handle Leave event of the control and set it back to previous selected input language.

In the below code, I set the input language to Persian when I enter TextBox1 and set it to previous language when I leave the control:

InputLanguage original;
private void textBox1_Enter(object sender, EventArgs e)
{
    original = InputLanguage.CurrentInputLanguage;
    var culture = System.Globalization.CultureInfo.GetCultureInfo("fa-IR");
    var language = InputLanguage.FromCulture(culture);
    if (InputLanguage.InstalledInputLanguages.IndexOf(language) >= 0)
        InputLanguage.CurrentInputLanguage = language;
    else
        InputLanguage.CurrentInputLanguage = InputLanguage.DefaultInputLanguage;
}
private void textBox1_Leave(object sender, EventArgs e)
{
    InputLanguage.CurrentInputLanguage = original;
}

To test the example you should have fa-IR as input language installed on your OS, otherwise it will set the language to default input language. You can use another culture input-language which you know installed on your OS.

Note: If you extensively need such feature in your forms, as an idea you can create an Extender Provider component providing an InputLanguage property. This way you can set the property at design-time. That's the way that components like ToolTip or HelpProvider works.