I wasn't aware of it, but the word has a long history of usage in English because there is no substitute.
OED: E. M. Forster, "Where Angels Fear to Tread," 1905. "The person who understands us at first sight, who never irritates us, who never bores....that is what I mean by simpatico.
OED: H. Sidgwick, "Memoir," 1864. The Frau Professorin was less 'simpatica.'

Webster's Collegiate: Being on the same wavelength (no date). First known date- 1864 (same ref. as the OED).