How to change default input language under tab language->Details programmatically?

I think that you can use SystemParametersInfo[^] API with action parameter SPI_SETDEFAULTINPUTLANG. :)

[UPDATE]
Hi, sorry for the late answer. :(
So, you need an example to change the default input language to English.
Then here is the example:

DWORD hKLEnglUS = 0x00000409;
if(!SystemParametersInfo(SPI_SETDEFAULTINPUTLANG, 0, &hKLEnglUS, SPIF_SENDCHANGE))
      TRACE(_T("Error code: %d"), GetLastError());

This works fine for me. :)

As a note I have to tell you that first I've tried using SystemParametersInfo with result returned from LoadKeyboardLayout but SystemParametersInfo failed with error code 998 (Invalid access to memory location).
After that I found that HKL is a pointer to DWORD that contains the the language identifier.
You can see the language identifiers table in this[^] MSDN page. As you can see from the example I'm using the constant 0x0409 (but with additional zeroes for the HIWORD part), which stands for English US.
I hope this helps.
:)
[/UPDATE]