private void fixLanguage(string whichLanguage) 
        {
            string useLanguage;
            switch (whichLanguage.ToLower() )
            {
                case "chinese":
                    useLanguage = "zh-CN";
                    break;
                default:
                    useLanguage = "en-US";
                    break;
            }
            InputLanguage language = InputLanguage.CurrentInputLanguage;
            if (language.Culture.Name != useLanguage)
            {
                foreach (InputLanguage lang in InputLanguage.InstalledInputLanguages)
                {
                    if (lang.Culture.Name == useLanguage)
                    {
                        InputLanguage.CurrentInputLanguage = lang;
                    }
                    System.Console.WriteLine(lang.Culture.Name);
                }
            }
            if (language.Culture.Name == useLanguage)
                this.ImeMode = System.Windows.Forms.ImeMode.On;
        }

Just a snippet I like; this can be called on GotFocus or LostFocus of a field, and it *attempts* to set the input language to the desired language.

setLanguage in WinForms

forms

C#