Going from Belarusian to Croatian/Serbian (Hrvatski) has a composite linguistic distance of 44% (heuristic estimate). Below is the dimension-by-dimension breakdown.
Shared vocabulary, cognates, and loanwords.
Morphology, syntax, word order, cases.
Shared sounds and phoneme inventory.
Cyrillic → Latin/Cyrillic
Indo-European ↔ Indo-European
The 5 languages with the lowest linguistic distance from Belarusian:
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