A great many expats manage to isolate themselves from the Japanese language. Often it's a combination of (a) circumstance - working in exclusively English-speaking environments, having only English-speaking colleagues and friends (both foreign and Japanese), and (b) desire - making no effort to learn any more than the most basic vocabulary and avoiding reading altogether. But still there's no substitute for living there and putting yourself in situations where you have to speak it.
Before I went to Japan (in the late 1980s) I took lessons, but it really only built up my passive knowledge. Without frequent opportunities to get some conversation practice with a real live human it was definitely uphill work. And if ever a language requires heavy practice, its Japanese. Of course there are always people who think it's something you can just pick up, but many newcomers have been disabused of that notion very quickly.
Of course speaking good Japanese is difficult enough, with its levels of politeness and its grammatical nuances and so on. But as a foreigner in Japan the locals usually cut you a lot of slack and tolerate what would otherwise be considered blunt and crude Japanese. in fact they're often highly impressed by even the most rudimentary sentences you utter in their language.
Reading and writing Japanese is a big undertaking of course. For that you have to become as a child again, forgetting everything you've grown up with in terms of how words are structured (syllabically rather than orthographically) and how they're represented visually, viz combinations of kanji - Japanicized Chinese characters - and two types of kana (syllabaries). There are no shortcuts,and it just requires frequent practice and reinforcement.
I used kanji cards when I started. They're just pocket-sized cards with a kanji on one side and the pronunciation and meaning printed on the other side, handy to carry around for whenever you've got a few minutes to spare. I also found it was much more effective to memorize kanji through writing practice rather than just trying to remember in my mind's eye what each kanji looked like. You are supposed to write each kanji with each particular "stroke" of the pen coming in a fixed order, and writing and rewriting a kanji many times in the correct order really helps to reinforce it.
One of the biggest hurdles is the way each kanji can have multiple pronunciations depending on whether it stands alone or whether it's in combination. Also, different combinations can result in one kanji having multiple pronunciations. For example 建 (referring to "building" or "construction") can be pronounced ken, kon, tate or date, depending on how it's used (what it's combined with in a word). You get a feel after a while for whether a particular combination requires a pronunciation derived from its original Chinese source (ken or kon in the earlier example) or a native Japanese pronunciation (tate or date). But this takes time. And it's always going to be messy, with odd and quirky pronunciations all over the place. (Keep in mind it's not easy for Japanese people either, and they have to spend a lot of classroom hours right up to the end of high school mastering all this.)
I used to get a lot of practice with hiragana (one of the two syllable-representing systems I mentioned earlier) when I walked on the street. Every Japanese car's number plate has a hiragana symbol on it, and in the course of a few minutes you can see hundreds of them. I found that a good way to get those hiragana fixed in my memory, as long as I didn't get hit by a moving vehicle or another pedestrian.
Also I made it a kind of hobby to learn the pronunciations of the kanji used in people's names. You see certain kanji used very often in surnames (山, 田, 川, 藤 and so on) and given names (子, 美, 介, 祐, 和 and so on). That was - for me at least - a fun way to learn some of the more commonly used kanji. It was (and still is) a good little way to break the ice when meeting Japanese people for the first time, asking if the kanji in their name is ... or ... or maybe ... They're usually pretty surprised when they get that sort of thing from a foreigner.
By the way, that -n- sound in arigatou is not universal. People in the Kanto area (around Tokyo) and in northern Japan often (but not always) nasalize the -g- sound and turn it into a soft -ng-. It's also something you hear out of the mouths of newscasters on NHK (Japan's equivalent of the BBC). But it's not done much at all in western Japan (Kyoto, Osaka etc) or in southern Japan. In fact in that part of the country it would sound a bit pretentious. I never nasalized the Japanese -g-, even when I lived up north, and nobody seemed to care.