The Greek spoken in Southern Italy sounds like it’s spoken by the Mario brothers. The Greek spoken in the Ukraine sounds soaked in vodka. And when I’m not in Greece, my dentals become alveolar: I sound like a caricature of “Uncle Nick from America”.
Basques live in Spain. The grammar has remained impervious to contact, which is cool. The lexicon presumably hasn’t, because that’s what happens when you’re a minority language.
My epiphany about accents came when I heard a performance of the 1830s Greek comedy Babel, which was about how Greek accents were incomprehensible. I’ve never heard Cappadocian Greek spoken. There is a Cappadocian character in the play. And damn if his accent didn’t sound Turkish.
Intonation and phonetics are subject to contact as well. Maybe a bit more than grammar: people’s identity doesn’t feel as compromised if they sound like their neighbours, because intonation and phonetics is somewhat less tangible than vocabulary and syntax.
Betcha the Basques across the border sound like Pepe le Pew. And that the German speakers in Alsace do too.