Going from Thai to Indonesian (Bahasa Indonesia) has a composite linguistic distance of 81% (heuristic estimate). Below is the dimension-by-dimension breakdown.
Shared vocabulary, cognates, and loanwords.
Morphology, syntax, word order, cases.
Shared sounds and phoneme inventory.
Thai → Latin
Tai-Kadai ↔ Austronesian
The 5 languages with the lowest linguistic distance from Thai:
A quick 3-question check between Thai and Indonesian. No signup.