Sorry, you cannot change the keyboard language in code on WP7.
Update
I did a bit of research on pulling the current language (Locale) from the app thread and came up with this post on in-app language selection in code. This looks like it was designed to localize text, however, I have a hunch the keyboard language will follow suit. Good Luck!
http://dotnetbyexample.blogspot.com/2011/07/mvvmlight-based-language-selection-for.html
Thread.CurrentThread.CurrentUICulture = new CultureInfo(CurrentLanguage.Locale);
Thread.CurrentThread.CurrentCulture = Thread.CurrentThread.CurrentUICulture;