@TyrantKoala - You should not be conflicted. The advice that “…colleges prefer students who are really good at one thing rather than decent at everything” is only applicable beyond a minimum number of core classes and/or a certain level of mastery in those core subjects. For the most selective schools (you state that you are considering the “best colleges around the country,” the expectations would include:
4 years of English
4 years of math (preferably through Calc)
4 years of science including a minimum of 3 lab sciences
4 years (or through level 4) of a foreign language
3-4 years of history/social science;
The “really good at one thing” comes into play when choosing electives, extracurricular activities and summer programs. They still want to see the core classes.
Carnegie Mellon and MIT seem to require only 2 years of foreign language, so you may be okay as far as they go, but you might be shooting yourself in the foot with other schools that you might be interested in.
What you did in 8th grade doesn’t matter unless you took French 2 and were able to get through French 4 by sophomore year of high school (i.e., the level matters more than the number of high school classes you took).