Going from Czech to Croatian/Serbian (Hrvatski) has a composite linguistic distance of 34% (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/Cyrillic
Indo-European ↔ Indo-European
The 5 languages with the lowest linguistic distance from Czech:
A quick 3-question check between Czech and Croatian/Serbian. No signup.