言而无信的英文：
[Literal Meaning]
words/aux./not/credit
to make a vain promise

[解释]
说话不算数，没有信用。

[Explanation]
to fail to keep faith

[例子]
你既然说过你要在八月底前写完报告，就不能言而无信。

[Example]
Since you have said you will complete the report before the end of the end of August, you cannot make a vain promise.

[英文等价词]
fail to keep faith
to go back on one's word
Breach of Promise.
to break one's word

Don't break your word if you want help.
如果你要人帮助你，就不可言而无信。