Носитель языка проверил локализацию.

Because язык is in the genitive case: языка.

The noun носитель often means bearer / carrier / holder of something, so Russian treats the thing “carried” as a dependent noun in the genitive:

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

So this is a standard expression, and носитель язык would be ungrammatical.

Языка is genitive singular of язык.

Its dictionary form is:

• язык = language / tongue

Here it becomes языка because it depends on носитель in the fixed phrase носитель языка.

This is similar to English patterns like:

• speaker of a language
• teacher of Russian
• owner of a house

Russian often uses the genitive where English uses of.

Yes. Literally, it is something like bearer of the language, but in normal usage it simply means native speaker.

So:

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

In your sentence, носитель языка is understood as a native speaker.

Проверил is the perfective past form of проверить, which focuses on a completed action.

• проверил = checked, verified, reviewed, and finished doing so
• проверял = was checking / checked in a process-oriented sense / used to check

In this sentence, the idea is that the native speaker completed the check of the localization, so проверил is the natural choice.

That is the normal marker for the past tense in Russian.

Russian past tense also agrees with gender and number:

• проверил = masculine singular
• проверила = feminine singular
• проверило = neuter singular
• проверили = plural

So проверил tells you the subject is grammatically masculine singular. It could refer to a man, or sometimes to a masculine noun phrase.

Usually, yes: проверил normally implies a male subject, because it is the masculine singular past tense form.

So this sentence most naturally means that a male native speaker checked the localization.

If the native speaker were female, you would usually say:

• Носитель языка проверила локализацию.

That said, in some contexts Russian can use masculine wording generically, but in a simple sentence like this, most learners should understand проверил as masculine.

Локализацию is accusative singular.

The dictionary form is:

• локализация

Because it is the direct object of the verb проверил, it goes into the accusative:

• локализация → локализацию

This change is normal for many feminine nouns ending in -ия:

• информация → информацию
• ситуация → ситуацию
• локализация → локализацию

Russian does not have articles like English a and the.

So:

• Носитель языка проверил локализацию.

can mean, depending on context:

• A native speaker checked the localization
• The native speaker checked the localization

Russian leaves that distinction to context.

Yes. Russian word order is more flexible than English because the case endings show each word’s role.

The neutral order here is:

• Носитель языка проверил локализацию.

But you could also say:

• Локализацию проверил носитель языка.
• Проверил локализацию носитель языка.

These versions may shift the emphasis:

• Локализацию проверил носитель языка. = It was the localization that a native speaker checked.
• Проверил локализацию носитель языка. = more literary or marked, often focusing on the action first.

The original sentence is the most straightforward and neutral.

The stress is:

• носи́тель языка́ прове́рил локализа́цию

A rough pronunciation guide:

• носи́тель = na-SEE-tyel
• языка́ = ya-zih-KAH
• прове́рил = pra-VYE-ril
• локализа́цию = la-ka-li-ZA-tsi-yu

Listening to native audio is especially helpful here, because Russian vowel reduction makes unstressed vowels sound weaker than their spelling suggests.

Grammatically, носитель is a masculine noun.

If you want to refer specifically to a woman, you may see:

• носительница языка

However, in many real contexts, especially formal or professional ones, people often still use носитель языка as a general term.

So:

• носитель языка проверил = male native speaker checked
• носитель языка проверила = the noun phrase stays the same, but the verb shows the speaker is female in actual usage in some contexts
• носительница языка проверила = explicitly female native speaker

For learners, the safest takeaway is that носитель is grammatically masculine, and the verb form helps show who is being talked about.