I’d tell my high-school self not to even take Japanese classes and just ask the teacher to help me learn on my own time. Japanese is best learned one-to-one, without picking up bad gaijin habits from classmates.
One connection I’ve been able to make with Japanese and Russian, at least, is sort of thinking of Japanese particles almost like Russian noun declensions. I know Polish has noun declensions similar to Russian. Is Polish loose with word order like Russian is? Figuring out each word’s function in a sentence has been a big part of the challenge for me in both Japanese and Russian.