But in the old Spanish, they were pronounced Shavier, Meshiko, and Teshas. So, back to Sonja’s side again. To the practical point, modern Spanish /x/ offers a choice for toki pona, /k/ for its velarity or /s/ for its fricativity. There is, at that point, no decisive reason for one rather than the other, so you use subjective factors. I don’t know Sonja’s subjective source here; maybe it’s just that ‘Mesiko’ “sounds better” than ‘Mekiko’ (it does to me). In any case, we are, as you note, stuck with it.