Going from Korean to Indonesian (Bahasa Indonesia) has a composite linguistic distance of 85% (heuristic estimate). Below is the dimension-by-dimension breakdown.
Shared vocabulary, cognates, and loanwords.
Morphology, syntax, word order, cases.
Shared sounds and phoneme inventory.
Hangul → Latin
Koreanic ↔ Austronesian
The 5 languages with the lowest linguistic distance from Korean:
A quick 3-question check between Korean and Indonesian. No signup.