These are common problems for languages with complex morphology. Finnish, Japanese, Turkish and Hungarian are closely related in this sense and share a lot of NLP research. While implementing spell checkers for these languages need a bit more effort than just compiling a list of words, it's far from an unsolved problem. AFAIK Zemberek[1] is a Turkish spellchecker that implements two-level finite state morphology. Als…
What's the problem with Japanese? It's highly regular language, so it should be easy to tokenize. AFAIR the whole language has maybe few irregular verbs, compared to few hundreds in English.
I can't speak Japanese, but if it is also morphologically rich, it should face similar problems.