What is your intended major?
To me, a C in calculus is a concern. The issue is not so much university admissions. The issue is that if you are going into anything related to math or engineering or physics then calculus is so very important. It is the basis for a LOT that someone in any of these majors is going to do in the future. I have used multi-variate calculus a lot, both in university classes and on the job. I would urge you to try hard for the rest of the year to pull this grade up. Whether calculus is as important for computer science will depend upon what type of CS job you would end up with. There are a lot of different types of jobs for a CS major.
And yes, a C in a very important core class might impact your chances at Dartmouth and Harvard.
I am not nearly as concerned about a B+ in Spanish. To me it seems like either you will be needing to use Spanish in the future and your Spanish will improve, or you won’t be using your Spanish in the future and it just won’t matter.
And of course Dartmouth and Harvard were reaches even before you got a C+ in calculus.