rezibukuro ga hituyou ka douka, seruhurezi no gamen de erabemasu.

かどうか means whether or not.

So:

• レジ袋が必要かどうか = whether a checkout bag is needed / whether you need a bag

This pattern is very common when Japanese wants to turn a yes/no question into a noun-like clause:

• 行くかどうか = whether someone will go
• あるかどうか = whether it exists
• 必要かどうか = whether it is necessary

In this sentence, the whole part レジ袋が必要かどうか is what you are choosing.

When かどうか is used inside a larger sentence, Japanese usually uses the plain form, not です / ます.

So:

• 必要かどうか = natural
• 必要ですかどうか = unnatural in normal Japanese

This is similar to how embedded clauses work in English: we do not usually keep the full surface question form inside another sentence.

So instead of something like whether is it necessary, Japanese uses the plain statement form plus かどうか.

Because レジ袋 is the thing that is being judged as necessary or not necessary.

• レジ袋が必要 = a checkout bag is necessary / you need a checkout bag

With 必要, the thing needed is often marked by が.

Compare:

• 水が必要です = Water is necessary / I need water
• 時間が必要です = Time is necessary / I need time
• レジ袋が必要です = A bag is necessary / I need a bag

To an English speaker, this can feel backwards, because English says I need a bag, but Japanese often frames it more like A bag is necessary.

No. Here 必要 does not mean that the bag itself is needy.

必要 is a na-adjective / adjectival noun meaning necessary.

So:

• レジ袋が必要 = a bag is necessary
• more naturally in English: you need a bag

Japanese often leaves out the person who needs it if that is obvious from context.

The subject is omitted because it is obvious from context.

In a self-checkout situation, the sentence naturally means:

• You can choose on the screen whether you need a bag.

Japanese very often leaves out subjects like you, I, or the customer when they are clear from the situation.

If you stated it explicitly, it might be something like:

• お客さんは、レジ袋が必要かどうか、セルフレジの画面で選べます。

But that is less natural than simply omitting it.

Here で marks the place or means where an action happens.

• セルフレジの画面で選べます = you can choose on the self-checkout screen

The idea is: the choosing takes place at/on the screen.

Why not に? Because に often marks a destination, time, or target, while で is used for the location of an action.

Compare:

• 店で買う = buy at the store
• 紙で書く = write with/on paper
• 画面で選ぶ = choose on the screen

It means the self-checkout screen.

Breaking it down:

• セルフレジ = self-checkout register
• の = linking particle, like of or an attributive connector
• 画面 = screen / display

So:

• セルフレジの画面 = the screen of the self-checkout
  or more naturally, the self-checkout screen

選べます is the polite potential form of 選ぶ (to choose).

• dictionary form: 選ぶ
• potential plain form: 選べる = can choose
• potential polite form: 選べます = can choose

So this sentence is saying not just choose, but can choose.

Compare:

• 選びます = choose / will choose
• 選べます = can choose

That can is important here.

The comma separates the long clause from the main part of the sentence.

• レジ袋が必要かどうか、 = whether you need a bag,
• セルフレジの画面で選べます。 = you can choose on the self-checkout screen.

Japanese commas are often used to make sentences easier to read, especially when the first part is long. It is not always strictly required, but here it is very natural and helpful.

必要 is usually treated as a na-adjective (or adjectival noun).

That is why you can say:

• 必要です = is necessary
• 必要な袋 = a necessary bag
• 必要かどうか = whether it is necessary

In this sentence, 必要 works like a predicate:

• レジ袋が必要 = a bag is necessary

So although it may look noun-like, functionally it behaves like an adjective in many cases.

Yes, something like レジ袋が要るかどうか、セルフレジの画面で選べます。 would be understandable, but 必要 sounds more natural in this kind of formal or instructional context.

Very roughly:

• 要る = need, more everyday and direct
• 必要 = necessary, slightly more formal or neutral

On signs, instructions, and store interfaces, 必要 is common because it sounds clear and impersonal.

レジ袋 means the bag you get at the register/check-out, usually a plastic shopping bag or store bag.

Breaking it down:

• レジ = register / checkout
• 袋 = bag

So it is not just any bag in general; it specifically refers to the bag offered at checkout.

Because many stores in Japan ask whether you want a bag, this is a very common word to see.