Japanese the language qua language is relatively easy from a language agnostic perspective. Formality levels are tricky, but the phonology is fairly simple (very few "weird" sounds) and the grammar is on the regular side.

The writing system on the other hand... IMO the worst used by any major language in the world. Graft a logographic system (already a bad choice to start) from an isolating language onto a highly synthetic verb system, necessitating a whole extra syllabary. Evolve numerous "readings" of each logogram, depending on etymology and history. Then tack on a 2nd syllabary for abstruse historical reasons.