Going from Basque to Swahili (Kiswahili) has a composite linguistic distance of 73% (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
Isolate ↔ Niger-Congo
The 5 languages with the lowest linguistic distance from Basque:
A quick 3-question check between Basque and Swahili. No signup.