D.C.
My daughter has attended the SSH in Arlington since it opened a couple of years ago. She loves it, and so do I. She also attended a summer camp at the Grapevine SSH a few years ago, and had a lot of fun there.
I can't say anything about the Flower Mound location, but the teachers at both locations I've used were/are great. They generally speak Spanish as their first language and English as their second. The instruction is all in Spanish, but they will help the students who don't fully understand - and kids pick up languages so quickly anyway.
I'll bet your daughter will love it. She may be uncomfortable for a week or so, but soon she will be singing songs in Spanish, counting, reciting los meses del año y los días de la semana, and won't want to go home at pick up time (which is a good thing!). And what a great thing for your daughter - learning about other cultures and languages.