What this means for learners
KOTOKO and Javanese share 22 sounds — roughly 61% of Javanese's inventory overlaps with KOTOKO. Shared sounds are ones a speaker already knows from their native language and will generally produce and perceive accurately without explicit training.
The 11 sounds found only in Javanese represent the greatest pronunciation challenge for KOTOKO speakers. The adult brain tends to map unfamiliar sounds onto the closest native equivalent — a process that produces the characteristic "accent" of a second-language speaker. Learning to hear and produce these sounds as distinct requires focused ear training, not just repetition.
Conversely, KOTOKO has 14 sounds not used in Javanese. Native Javanese speakers learning KOTOKO will face the mirror-image challenge with these sounds.
Phoneme inventories from PHOIBLE. Data reflects one documented inventory per language; some variation exists across dialects and sources.