I had a 3 week trip to Malaysia years ago, but it wasn’t January/February. The east and west coasts have different wet and dry seasons - my trip focused on the less developed east coast for this reason.
We started with 5 nights in Singapore - we should have added a couple of nights extra including getting over jet lag. Moving from the highly developed, very western Singapore to the much more sedate Malaysia was a complete culture shock. We swapped traffic noise for the sounds of the mosques calling within a couple of miles.
We took a bus across the straits and picked up our hire car in Johor Bahru and headed up the east coast. We hadn’t booked accommodation for this part of our trip, but we took recommendations from Lonely Planet. We had a few nights further up the coast on Redang island then headed to Taman Negara national park for some wildlife spotting in the rain forest and boat trips on the river. After retrieving our car, we went to the Cameron Highlands, the tea plantation area, which was much colder, being at altitude. Our car only had A/C, not heating so we had to pile several layers of clothes on to keep warm.
We had a few nights in KL before returning to Singapore.
I can’t recall meeting many locals - in rural parts, many didn’t speak English.
Indonesia has developed significantly since I was there about 20 years ago. The islands vary significantly, so I suggest that you need to check out a guide book to establish which would suit best. I covered Bali, Lombok and eastern Java. Lombok was far less developed than Bali at the time, but even today, if you focus further north, you can avoid the party hotspot around Kuta and Sanur. The mountains and paddy fields are worth visiting.
Java freaked me out as the locals kept coming up to me and stroking my hair as it wasn’t black like theirs - this may have changed these days. There are some interesting sights such as the temple at Borobudur, but you need to visit early as it’s very hot there. Nothing was possible between about 10am and 4pm due to the heat.