japanese and austronesian

I've read that one of the most popular theories on the origin of the Japanese language, especially in Japan, is that it's a hybrid of Altaic (Mongolian, Turkish, and likely Korean) and Austronesian (Indonesian, Tagalog, and the Polynesian tongues). I saw one forum about the Japanese language where a guy remarked on how he thought certain Indonesian customs were rather similar to those in Japan, and showed what he thought were similar grammatical customs (i.e., doubling a word to make a plural). But I don't know either Japanese or Indonesian, so could anyone who does know those languages state their opinons?

Yes to the Altaic. The other component is the Jomon language which has some affinity with some Austronesian altho I think nothing hs been proven. Ethnically the japanese are about 70% Northern Asian and 30% Jomon. But the Jomon also came to Japan from North asia . But people moved around in those days, what with Ice Ages waxing and waning, continental landbridges, game moving around etc. There is a theory that some of the Native Americans have Jomon DNA in them.