>>539
You can “master” Japanese just as well as you can “master” English. It takes a lot longer because most of were not raised in a kanji using culture.
In five years time I have managed JLPT1, good speaking and listening skills with long-term practical experience, and I am capable of reading research articles within my field in Japanese and discussing my own research and thoughts in Japanese. Give me another five years, and I will be able to speak fluently and be capable of reading and writing anything I want in Japanese without needing a dictionary any more than I do for English or my native language.
But I do sometimes need a dictionary for my English as well as my mother tongue, so if by that definition I have not “mastered” those languages, than Japanese cannot be mastered as well. By that definition most Japanese have not done so either.