Going from Zulu to Swahili (Kiswahili) has a composite linguistic distance of 44% (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 → Latin
Niger-Congo ↔ Niger-Congo
The 5 languages with the lowest linguistic distance from Zulu:
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