Going from Basque to Croatian/Serbian (Hrvatski) has a composite linguistic distance of 83% (heuristic estimate). 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
Isolate ↔ Indo-European
The 5 languages with the lowest linguistic distance from Basque:
A quick 3-question check between Basque and Croatian/Serbian. No signup.