What's the original Chinese to “To one who has seen too little of the world, everything is strange.”?
中国古典文学 has the preface.
Here's the first paragraph:

初刻拍案惊奇
凌濛初
序
语有之：“少所见，多所怪。”今之人，但知耳目之外，牛鬼蛇神之为奇，而不知耳目之内，日用起居，其为谲诡幻怪非可以常理测者固多也。昔华人至异域，异域咤以牛粪金；随诘华之异者，则曰：“有虫蠕蠕，而吐为彩缯锦绮，衣被天下。”彼舌挢而不信，乃华人未之或奇也。则所谓必向耳目之外索谲诡幻怪以为奇，赘矣。

The translation of “To one who has seen too little of the world, everything is strange.” is:

少所见，多所怪

There is also an entry for this in ABC Proverbs which says:

Lit [Things (which are)] seldom seen [are felt to be] strange.
Note Cf. the preface to Chū Kè Pāi'àn Jīngqì; now more commonly said shǎo jiàn, duō guài; see also the preceding entry.

Like the definition mentions the more, modern, common variant of this phrase is 少见多怪 which ABC defines as:

The less one has seen, the more one marvels.


ABC Proverbs also contains the old phrase with an added sentence:

少所见多见怪，睹橐驼谓马肿背。
Lit [One who has] seen little regards many things as strange; [(s)he] sees a camel [and] calls [it a] horse with a hunchback.