It is but a mirage......إنما هو كبرق الخلب

 It is but a mirage. It is said: "a mirage," and "a mirage without rain," both referring to lightning that is deceptive, like a flash of lightning. "A mirage" also refers to a cloud that brings no rain. So, when it is said, "a mirage of a cloud," it means a mirage of a cloud that is deceptive. This is said of someone who makes a promise and then breaks it.

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“All flash and no substance.”

Meaning: Just a shine without substance or benefit.

“All sizzle and no steak.”

A well-known American proverb, meaning: Much ado about nothing.

“Empty promises.”

Meaning: Empty promises.

“Much cry and little wool.”

An old proverb meaning: Much noise and little benefit.

“A flash in the pan.”

Said of something that shines quickly and then disappears without a trace.

And the closest to the spirit of the Arabic proverb in terms of deception by shine is:

“All flash and no substance.”

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 يقال: برق خلب وبرق خلب بالإضافة، وهما البرق الذي لا غيث معه كأنه خادع، والخلب أيضاً، السحاب الذي لا مطر فيه. فإذا قيل، برق الخلب، فمعناه، برق السحاب الخلب يضرب، لمن يعد ثم يخلف ولا ينجز.