Wah - Marina's Korean is way better than mine. I will have to study this notebook entry carefully to learn from her.
Still, I'd like to make some comments from the perspective of a native English speaker and ask for compassionate correction if I get this wrong.
First of all, 산 means "mountain" in Korean so I think you should either say Seoraksan or Mount Seorak but not Mount Seoraksan which would mean "Mount Seorak mountain".
Secondly, in English, "maple" is a kind of tree. So, if you say "because that was during the spring, I was unable to see the maples", you've said something that is not true. Of course, you were able to see the maple trees. It's just that the leaves of the maple trees were green during the spring, not red, brown and yellow as they are in the fall.
So... despite the fact that Koreans don't say "turning of the leaves" or "fall color change", you can't just say "the maples" because it won't make sense in English.
This kind of thing occurs from time to time when translating from one language to the other. Sometimes, there just isn't a good literal translation that will communicate the idea and you have to find the expression in the second language that expresses the idea of the original text even if it is not a literal translation.
I would use a phrase like "turning of the leaves" or "fall change of colors" to capture the meaning of the Korean text.
Notes:
I am unsure weather I should translate it to "maples" or "colour changes of the leaves".
I am also unsure of my translation of the sentence "이번 여행이 끝난 후 20일에 한국을 떠날 거예요".
Your translation is almost perfect. Wow! Really great. AWESOME!:D
1. Maple is okay. We ususally express in "Maple". Korean don't usually say like "colour changes of the leaves".
2. "이번 여행이 끝난 후 20일에 한국을 떠날 거예요" means like this. After ending this trip, I will leave Korea on 20. So your translation needs a little correction.