Stupid mistake on college app. Please help!

Well, I applied to University of Maryland before Nov, and would get decision by Feb First. However, just two days ago, I found out that I accidentially put English as my first language, which should be Korean instead (I have US Citizenship, though.) The problem is my teacher’s recommendation clearly indicates that I am working hard to improve my English (Well, please don’t misunderstand that she showed me that way after submission.)
Well, in my opinion, that sounds not that big deal, for I think the reason they ask for first language is to clarify my English proficiency, and since my new SAT English score, 680, is high enough to waive Toefl, I think, that’s enough to prove my English Proficiency. But, in a case, I’m worried about if that mistake would eventually ruin my chance of getting in that University. How do you think about it?

It shouldn’t be a problem.
That said, why don’t you send a clarifying email to them explaining your error,
and ask them to make the adjustment to your application.