Dear Ong Yong,

> As for Xuanzhuang, it is Xuanzhuang, which is his ordained name.

[snip]

> I have no idea how exactly English/European materials come to
> write/call him as Xuanzang.

Actually, Xuanzang *is* the correct form. I have now checked this in
several dictionaries, including the Hanyu Da Cidian (the most authoritative
single character Chinese dictionary). They all state that the second
component in his name is read "zang". Your "zhuang" is mentioned, but Hanyu
Da Cidian states that this reading is colloquial/dialectical.

Best wshes,
Stephen Hodge