Indonesian is not so difficult but you still have memorize lots of vocab.
Same as learning any language. Memorize three new words a day and in a year, you know about 1,000.
The hardest part in learning languages is the grammar. The second hardest is correct pronunciation. The third is trying to speak like a local, (depending where you are).
Indonesian grammar is fairly straight-forward, (unlike English), so being able to be understood is not all that difficult. On the other hand, engaging in conversations dealing with complex topics, (such as philosophy, for example), requires a much more advanced grasp of Indonesian than the mere basics. Sophisticated Bahasa Indonesia requires considerably more time and effort than being able to ask
how much is that coconut? in a market.
Easy to learn when you live there, when you have to function. Book/on line learning does not work.
It may not "work" 100%, but it certainly gives you a workable foundation to build on.
I mulled over "knalpot" for a while the other day..... why the indonesian word for exhaust came to me is a mystery.
I've had the same experience many times. (Not only about "knalpot"; "cat oven" keeps popping up for me.)
When I go back to my western country for a visit, I notice that I get tongue-tied quite frequently speaking "real" English. I truncate sentences, change the word order around, and when shopping, try to bargain, etc. "
Berapa those prawns very big, again? Is that a morning price"
My mother, (now 86 years old), could speak 16 languages in her hey-day. Her worst one was English. My father, (long dead), could speak 8 languages. His worst one was English. I could speak 3-4, sort of, when I was six. I managed to get university degrees in English language and literature, but became a psychologist instead.
Knowing too many languages can drive anyone bonkers. Maybe that's why I became interested in psychology in the first place.
IMHO, those mp3 Bahasa Indonesia lessons that
markit pointed to are very worthwhile.