Korean is the easiest language to learn for us Japanese speakers. The reasons being 1 The word order is the exact same as Japanese 2 The grammar is similar 3 A huge part of the vocabulary is shared through kanji 4 The culture is similar. What we already know through Japanese can be used in Korean. Even if you are a complete beginner starting from learning hangul, you can get to a high level in Korean in a short period of time. Its safe to say that if you already know the basics of the Japanese language you will improve Korean quickly.