The use of "x" in English to represent the "ks" sound comes from the Greek letter "xi" (ξ). 
This convention was established when Greek words were adopted into Latin and then into English.

Inglish also uses -x wherever English would.

Examples:
    axiom: From Greek "ἀξίωμα" (axioma), meaning "that which is thought worthy or fit."
    syntax: From Greek "σύνταξις" (syntaxis), meaning "arrangement" or "order."
    paradox: From Greek "παράδοξον" (paradoxon), meaning "contrary to expectation."

Inglish:
    axiom
    sỳntax
    pâradox