Going from Javanese to Malay (Bahasa Melayu) has a composite linguistic distance of 38% (curated lexical data). Below is the dimension-by-dimension breakdown.
Shared vocabulary, cognates, and loanwords.
Morphology, syntax, word order, cases.
Shared sounds and phoneme inventory.
Latin/Javanese → Latin/Arabic
Austronesian ↔ Austronesian
The 5 languages with the lowest linguistic distance from Javanese:
A quick 3-question check between Javanese and Malay. No signup.