Для локализации нужен носитель языка.

Because the preposition для always takes the genitive case.

• локализация = nominative singular
• для локализации = for localization / for the purposes of localization

So this is a very common pattern:

• для работы = for work
• для проекта = for the project
• для локализации = for localization

Нужен agrees with the noun that is needed: носитель.

Since носитель is:

• masculine
• singular

the correct form is нужен.

Compare:

• нужен носитель = a native speaker is needed
• нужна помощь = help is needed
• нужно время = time is needed
• нужны специалисты = specialists are needed

So the form depends on the noun after it.

Носитель is in the nominative singular. It is the thing being described as needed.

In this kind of sentence, Russian often uses:

• X нужен = X is needed

So here:

• носитель = nominative singular
• языка = genitive singular, dependent on носитель

The phrase носитель языка literally means something like bearer/carrier of the language, which is the standard Russian way to say native speaker.

Because носитель commonly takes the thing “borne” or “possessed” in the genitive.

So:

• носитель языка = native speaker of a language
• literally: bearer of the language

This is just how the expression works in Russian. You should learn носитель языка as a set phrase meaning native speaker.

In present-tense Russian, the verb to be is usually omitted.

So Russian says:

• Для локализации нужен носитель языка.

where English would say:

• A native speaker is needed for localization.

This is completely normal. Russian usually does not use a present-tense equivalent of is/are in simple statements like this.

Yes. Russian word order is flexible, and changing it usually changes the focus or emphasis, not the core meaning.

For example:

• Для локализации нужен носитель языка.
  Focuses first on for localization.

• Носитель языка нужен для локализации.
  Starts with a native speaker as the main topic.

Both are natural. The original sentence sounds like it is answering the question What is needed for localization?

Usually, yes. In normal usage, носитель языка means native speaker.

It is the standard neutral expression in Russian.

Examples:

• носитель русского языка = a native speaker of Russian
• носитель английского языка = a native speaker of English

Sometimes Russian speakers may shorten it to just носитель if the language is already clear from context.

All of them can express necessity, but they work differently.

нужен / нужна / нужно / нужны

These agree with a noun and mean is needed / are needed.

• Нужен носитель языка. = A native speaker is needed.
• Нужна помощь. = Help is needed.

надо / нужно

These are more impersonal and often mean it is necessary or one needs to.

• Для локализации нужно найти носителя языка. = For localization, it is necessary to find a native speaker.
• Надо проверить текст. = The text needs to be checked / We need to check the text.

So in your sentence, нужен is used because the sentence is centered on the noun носитель языка.

Russian has no articles, so it does not distinguish directly between a native speaker and the native speaker the way English does.

The exact meaning comes from context.

So:

• нужен носитель языка can mean a native speaker is needed
• in another context, it could mean the native speaker is needed

In this sentence, English usually translates it as a native speaker.

Yes. In modern Russian, локализация is commonly used in the sense of:

• adapting software
• adapting games
• adapting websites
• adapting content for a specific language or market

So Для локализации нужен носитель языка is a very natural sentence in a translation or product context.

It does not mean physical location here.

Yes. That would also be correct.

• нужен = needed
• необходим = necessary / essential

необходим sounds a bit more formal or stronger.

So:

• Для локализации нужен носитель языка. = A native speaker is needed for localization.
• Для локализации необходим носитель языка. = A native speaker is necessary for localization.

Both are grammatical and natural.