[Taberu]

I'm currently learning all three languages, Japanese being my second language; Korean, my third; Chinese, my fourth and English, my native.

Having learnt Japanese for quite some time before starting Korean made the process easier. The grammar is almost identical, though there are some differences here and there, and the words of Chinese and foreign origin are similar sounding. Although the alphabets of each language differ, the sounds are relatively the same and a Korean person should be able to pronounce Japanese almost perfectly, while a Japanese person might have trouble getting rid of the tendency to add a vowel after a consonant.

I've been learning Chinese for a little over three months, but the grammar is vastly different from Korean and Japanese; much closer to English, in fact. Mandarin generally uses simplified characters in writing, whereas Japanese will use a mix of simplified and traditional characters. When learning both languages, the distinction may be difficult to make at first.

Mandarin and Japanese pronunciation is extremely different due to the fact that Japanese doesn't have the sounds that Mandarin does, so similar sounds need to fill the space. Also, the Chinese origin words were brought into Japanese at a time when both languages were vastly different from what they are today (and the fact that there are many different Chinese dialects with different sounds must be factored into the equation).

Simple Answer: Japanese and Korean are closer as languages than Japanese and Chinese are.