MY best friend is Japanese. The language fluency is extremely difficult for English speaking adults to transition into, because the way they think and organize thoughts is so radically different than that of westerners. Far more than most are prepared for when they take on the challenge of learning Japanese.
They also tend to have significantly higher I.Q. ranges, and recall capacity. Therefore, the language is very intricate with specific and precise terms, and many many contextual factors that determine how each word is to be taken. It can get quite daunting for westerners to adjust to.
If a person doesn’t have close friendships or otherwise spend a lot of time interacting with native Japanese humans, it’s almost impossible to become fully situated. Not likely to be able to learn just from observation. The simple fact is, we tend to need someone there with us to ask questions every step of the process, or we end up very misguided by resting on our own (western oriented) assumptions.