If you need a drop-in replacement of Java BigDecimal, I’ve ported the Android implementation to Kotlin, usable for Kotlin/Native (particularly for iOS). It cannot support Kotlin/JS though, as it depends on BoringSSL for the actual BIGNUM implementation: GitHub - kendy/Kotlin-Native-BigDecimal: Kotlin/Native BigDecimal implementation
Having said that, in the 2nd part of your question, you ask for native language support; so actually it is unclear to me what do you want to achieve? What is the problem you want to solve?