Kalau saya rajin belajar, saya mau jadi guru bahasa Indonesia.

Kalau means if here.

So Kalau saya rajin belajar, ... means If I study diligently, ... or If I am diligent in studying, ...

A useful thing to know:

• kalau is very common in everyday Indonesian
• it can also sometimes feel like when depending on context
• a more formal alternative is jika

Because each clause has its own subject:

• Kalau saya rajin belajar = If I study diligently
• saya mau jadi guru bahasa Indonesia = I want to become an Indonesian-language teacher

Indonesian often repeats the subject when it helps keep the sentence clear. In English we do the same thing:

• If I study hard, I want to become...

Leaving out the second saya would usually sound incomplete here.

Rajin means diligent, hardworking, or regular about doing something.

So rajin belajar literally means something like:

• diligent in studying
• studying regularly
• studying hard

It is a very natural combination in Indonesian.

It is basically an adjective, but Indonesian lets words like this work in ways that English might express with either an adjective or an adverb.

So:

• saya rajin = I am diligent
• saya rajin belajar = I am diligent about studying / I study diligently

You do not need a special -ly form like in English.

Because Indonesian often does not use a verb like to be in sentences with adjectives.

So:

• saya rajin = I am diligent
• dia senang = he/she is happy
• mereka sibuk = they are busy

This is very normal Indonesian grammar.

Belajar usually means to study or to learn in a general sense.

That fits perfectly in:

• rajin belajar = study diligently

Mempelajari is more like to study something specific.

For example:

• Saya mempelajari tata bahasa Indonesia = I study Indonesian grammar

So in your sentence, belajar is the natural choice.

Here mau most naturally means want to.

So:

• saya mau jadi guru bahasa Indonesia = I want to become an Indonesian-language teacher

In other contexts, mau can sometimes suggest something like going to or about to, but want to is the best reading here.

If you wanted a clearer future meaning, you might use:

• akan = will
• bisa = can / be able to

For example:

• Kalau saya rajin belajar, saya akan menjadi guru bahasa Indonesia.
• Kalau saya rajin belajar, saya bisa menjadi guru bahasa Indonesia.

Both can mean become.

• jadi is shorter and more conversational
• menjadi is a bit more formal or written

So:

• saya mau jadi guru = natural, everyday Indonesian
• saya mau menjadi guru = also correct, slightly more formal

Both are fine.

Because Indonesian does not have articles like a, an, and the.

So:

• guru can mean a teacher, the teacher, or just teacher, depending on context

In your sentence:

• jadi guru = become a teacher

This is completely normal.

It means a teacher of the Indonesian language.

So bahasa Indonesia describes what kind of teacher:

• guru bahasa Indonesia = Indonesian-language teacher
• guru bahasa Inggris = English teacher
• guru matematika = math teacher

This does not primarily mean a teacher who is Indonesian. It means a teacher who teaches the subject Indonesian.

In a normal sentence, bahasa is usually lowercase and Indonesia is capitalized:

• bahasa Indonesia

That is because Indonesia is a proper name.

So your sentence is correctly written as:

• guru bahasa Indonesia

However, if it is the name of a school subject or a formal title, you may also see:

• Bahasa Indonesia

For an ordinary sentence like this one, bahasa Indonesia is fine.

Yes, the comma is appropriate here.

When the if-clause comes first in Indonesian, it is standard to separate it from the main clause with a comma:

• Kalau saya rajin belajar, saya mau jadi guru bahasa Indonesia.

This is similar to English:

• If I study hard, I want to become an Indonesian teacher.

If the main clause comes first, the comma is often not needed.

Yes.

You can say:

• Saya mau jadi guru bahasa Indonesia kalau saya rajin belajar.

This is grammatical, but the emphasis changes a little.

• Starting with kalau... puts the condition first
• starting with saya mau jadi... puts the main idea first

Both are understandable.

Yes.

• kalau = common, everyday, conversational
• jika = more formal, more written

So this version is also correct:

• Jika saya rajin belajar, saya mau menjadi guru bahasa Indonesia.

That sounds a bit more formal than the original.

It is grammatically understandable, but the exact meaning depends on what you want to say.

With mau, the sentence literally says that under that condition, you want to become a teacher. That can sound a little unusual, because studying diligently does not normally cause the desire itself.

If you mean If I study diligently, I can/will become an Indonesian-language teacher, these may sound more natural:

• Kalau saya rajin belajar, saya bisa jadi guru bahasa Indonesia.
• Kalau saya rajin belajar, saya akan menjadi guru bahasa Indonesia.

If you mean I want to study diligently so that I can become an Indonesian-language teacher, then this is very natural:

• Saya mau rajin belajar supaya bisa jadi guru bahasa Indonesia.

So the original sentence is understandable, but there may be a more natural version depending on your intended meaning.