塞翁失马的英文：
[Literal Meaning]
frontier/old man/lose/horse
An old man living at the frontier lost his horse.

[解释]
人的祸福可以变化，好事坏事在一定条件下可以互相转换。

[Explanation]
A loss may turn out to be a gain.

[例子]
虽然你没有买到那个股票，但是塞翁失马、焉知非福，也许过两天股市就跌了。

[Example]
You failed to buy that stock, but it may be a blessing in disguise - the market prices might drop in a couple of days.

[英文等价词]
a blessing in disguise

"Nothing so Bad, as not to be good for something"
塞翁失马,安知非福

No great loss without some small gain
塞翁失马,安知非福

On one hand,loss imply gain;on the other hand,gain imply loss.
塞翁失马，安知非福;塞翁得马，安知非祸。