It can always be sure that when you see a woman wearing scarf in Japan, she must from Malaysia or Indonesia. Just strike a conversation in BM and you will be able to know either Indo or Malay. I have started a few conversations (always with BM first) with a few people wearing scarf and it turned out all of them are from Malaysia.
In Ikebukuro while waiting bus back to Urasa, saw a lady wearing scarf talking to a guy in Bahasa Malaysia (not Indonesia). Ah... Malaysian.
It turned out we came back on the same flight and the guy, Asryn was staying in a town just 40 mins away from Urasa. He was in Japan for the past 6 years studying and working and went back to get married and brought his wife back to Nagaoka. Wow... what a story.