Actually, I looked at the DLC files and it's not juts unlock code, the DLC have the lua scripts and dialogue text in them.DLC is encrypted unlock code , jap DLC can't modify work on asia version , asia DLC not open sale at now
Furthermore, it turns out that the DLC are available for SEA version, but only on the South Korean and Hong Kong online store (NOT eShop) where you buy a voucher that can them be redeemed ingame.
Here's the store links for Korean and Hong Kong online stores with the DLC:
Hong Kong: https://www.whatsmall.com/cart/shopping?actionType=get&id=3617
South Korea: https://support.nintendo.co.kr/onlineStore.do?cmd=view&idx=357&categoryCode=Switch_DL
I can confirm the DLC is multilanguage on the Korean store and has Chinese/Korean/English, here's a screenshot from my Switch with the DLC downloaded and installed from Korean store: