天不怕, 地不怕，只怕美國人講國話 is a modified version. The original: 天不怕, 地不怕，只怕講鬼話.
The original version means nothing is worst than speaking English. The modified version is in reversed meaning: nothing is worst than American speak Mandarin.
漢語, 中國話,中文 means Chinese Language
普通話, 國語, 國話 means Chinese-Mandarin
The original version is a self expression of the Chinese speaking person. The reversed version is asked by the American which means that the Chinese is insulting the American, All but not most Amerian speak very poor mandarin especially in pronounciation of the Chinese words in mandarin.