kaigai de tukatta ka douka ni kakawarazu, roominguryoukin wo kakuninsuru.

海外で means overseas / abroad as the place where something is used.

The particle で marks the location where an action happens. Since 使う means to use, 海外で使った means used it overseas.

• 海外で使う = use it abroad
• 海外に行く = go abroad

So で is used because this is the place of the action, not a destination.

Japanese often leaves out things that are understood from context.

So 使った literally just means used or used it, but the sentence does not explicitly say what was used. In context, it is probably something like:

• a phone
• mobile data
• roaming service
• a SIM
• the device/service being discussed

A natural English translation often has to add something like your phone or the service, even though Japanese does not say it directly.

Because 使ったかどうか means whether someone used it or not.

The sentence is talking about whether the use happened, so the past form 使った is natural.

• 使うかどうか = whether someone will use it / whether to use it
• 使ったかどうか = whether someone used it

Here the speaker is referring to possible past use overseas.

かどうか means whether or not.

It turns a clause into something like an embedded yes/no question.

So:

• 使ったかどうか = whether it was used
• literally: did use or not?

This is a very common pattern:

• 行くかどうか分からない = I don't know whether I will go.
• 正しいかどうか確認する = check whether it is correct.

In your sentence, 使ったかどうか is the idea of whether it was used overseas.

Because にかかわらず is a fixed grammar pattern meaning regardless of or irrespective of.

So you should think of this as one chunk:

• 〜にかかわらず = regardless of 〜

That means:

• 使ったかどうかにかかわらず = regardless of whether it was used

The に is not doing something special on its own here; it is part of the set expression.

〜にかかわらず means regardless of 〜, whether or not 〜, or irrespective of 〜.

It says that the result in the main clause stays the same no matter what the condition is.

So here:

• 海外で使ったかどうかにかかわらず、ローミング料金を確認する。
• Regardless of whether it was used overseas, check the roaming charges.

The point is: you should check the roaming charges in either case.

They are similar, but not exactly the same in feel.

• 使ったかどうかにかかわらず = regardless of whether it was used
• 使っても使わなくても = whether you use it or not / even if you use it or do not use it

かどうかにかかわらず sounds a bit more formal and abstract. It is especially common in notices, instructions, contracts, customer support language, and written explanations.

使っても使わなくても sounds more direct and conversational.

So in a notice about charges or service conditions, かどうかにかかわらず fits very naturally.

Because Japanese instructions and headings often use the dictionary form in a manual-like or notice-like style.

So 確認する here can function like:

• Check ...
• Please check ...
• Be sure to check ...

Even though it is not explicitly polite, it can still sound like an instruction on a webpage, form, app screen, or help article.

Compare:

• 確認する = check / to check / check it
• 確認してください = please check

The first is common in bullet points, labels, and official instructional writing.

Yes. Japanese often omits the subject when it is clear from context.

This sentence does not say you, but the meaning is probably something like:

• You should check the roaming charges
• Please check the roaming charges

Depending on context, it could also mean a general instruction rather than directly addressing one person.

By itself, it simply means check the roaming charges.

Japanese does not specify exactly what kind of checking unless context makes it clear. It could mean:

• check the rate before traveling
• check whether charges apply
• check how much was charged
• check the roaming fee details

So the exact nuance depends on where the sentence appears.

It breaks down like this:

• 海外で = overseas / abroad
• 使った = used
• かどうか = whether or not
• にかかわらず = regardless of
• ローミング料金を = roaming charges
• 確認する = check

So the structure is:

[Regardless of whether it was used overseas], [check the roaming charges].

In other words:

• subordinate clause: 海外で使ったかどうかにかかわらず
• main clause: ローミング料金を確認する

Yes. That is a very natural translation.

Since Japanese leaves out the object and subject, English usually has to supply them. So depending on context, good translations might include:

• Whether or not you used it abroad, check the roaming charges.
• Regardless of whether you used your phone overseas, check the roaming charges.
• Check the roaming charges whether or not the service was used abroad.

All of these can be correct depending on what it refers to in the surrounding context.

The comma helps separate the condition part from the main instruction.

• 海外で使ったかどうかにかかわらず、 = regardless of whether it was used overseas,
• ローミング料金を確認する。 = check the roaming charges.

Japanese commas are often used more flexibly than English commas, but here it makes the sentence easier to read by marking the boundary between the introductory clause and the main clause.