If the text is popularizing, I'd go with chi, but since somebody or some editor made the decision to go with Pinyin romanization scheme, it would best be covered in the notes of the translator at the beginning. Many books which I have read make a compromise between official transliterations and using popular/common spellings. So, in a book on Russian culture, you might see names like Ejkhenbaum or Shklovskij, but Tchaikovsky (rather than Chajkovskij). Your publisher and/or their editorial board may have the last say.

Ceci n'est pas un seing.