I don’t normally include language schools here, but Middlebury is famous for its summer programs. While I have not experienced one myself, I have heard great things about them over many years from people I trust. 

The typical summer program at Middlebury Language Schools lasts six to eight weeks, and it is a total immersion experience. Here’s the pledge you have to sign to participate in one:

In signing this Language Pledge, I agree to use ______________ as my only language of communication while attending the Middlebury Language Schools. I understand that failure to comply with this Pledge may result in my expulsion from the School without credit or refund.

They are not kidding around.

Programs are available in Arabic, Chinese, French, German, Hebrew, Italian, Japanese, Portuguese, Russian, and Spanish.