この商品は、要冷蔵で五日間の消費期限です。って英語でなんて言うの？

Both of these phrases informs the person that the product will not go off if it is kept refrigerated (in the fridge). We call things that have gone past their expiry date - gone off.
"Euw, this milk has gone off" - It would smell horrible and probably make us gag.
"This doesn't taste right, I think it's gone off".

Many products nowadays have to be kept refrigerated in order to stay fresh.

"This product will last five days if kept refrigerated."

'to last' = this means to remain fresh and edible (in terms of foodstuffs)

"If you keep this product in the fridge, it will stay good for at least five days."

'to stay good'= this means to stay fresh and have a good quality that can be eaten

"Oh no! I didn't put the milk in the fridge, now it's gone bad and smells awful!"

'to go bad'= when a food product is expired and no longer can be eaten, when it starts to rot.

When talking about keeping/storing food then you have to be careful as you could make yourself ill if something isn't stored correctly so you should tell someone how to store the item
'in the fridge' means to place the item in the fridge
'refrigerated' means to be cold when stored in the fridge

The above sentences are different ways to say the same thing to a customer. If you would like to emphasize the need for the product to be cold/cool, you can use the phrases, "in the fridge," "kept refrigerated," or "in the refrigerator." If you would like to emphasize the life expectancy of the product, you can use the phrases, "it will last for," "it is good for," "it's usable for," "it will expire after," and then state the time period (five days).

The working week is something that lasts for 5 days in most countries. And in saying that the food would not last the working week the person in London would understand that it means 5 days.

"This food will not last long if you do not refrigerate it. If you do it will last 5 days or more.

This is giving the full description of what is happening with the food it is saying that it will spoil if it not put into the fridge. Then the opposite information is given.

You can use phrase like "last for..." or "kept for..." to talk about how long a product will last, while it is in the fridge (its expiration date).

Probably, in a confectionary store, if you want to give details of the 'best before' date, it is advisable just to state the facts as in the first example sentence above.
If you want to give more personal advice, then the second sentence may be more appropriate.
If it is a refrigerated or frozen item then it is probably not necessary to mention that it should be kept in a fridge. That is obvious! Eg: milk, yogurt, cheese etc.