What this means for learners
RORO and Tongan; Tonga share 10 sounds — roughly 59% of Tongan; Tonga's inventory overlaps with RORO. Shared sounds are ones a speaker already knows from their native language and will generally produce and perceive accurately without explicit training.
The 7 sounds found only in Tongan; Tonga represent the greatest pronunciation challenge for RORO 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, RORO has 4 sounds not used in Tongan; Tonga. Native Tongan; Tonga speakers learning RORO 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.