Going from Dutch to Basque (Euskara) has a composite linguistic distance of 81% (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
Indo-European ↔ Isolate
The 5 languages with the lowest linguistic distance from Dutch:
A quick 3-question check between Dutch and Basque. No signup.