Ako kasniš, javi mi se odmah.

Yes. Ako is the normal Croatian word for if in conditional sentences.

In Ako kasniš, javi mi se odmah., the structure is:

• Ako kasniš = If you are late / if you’re running late
• javi mi se odmah = let me know immediately / contact me right away

So ako introduces the condition.

Kasniš is the 2nd person singular present tense form of kasniti (to be late, to run late).

The verb forms are:

• ja kasnim = I am late
• ti kasniš = you are late
• on/ona/ono kasni = he/she/it is late

Since the sentence is talking to one person informally, Croatian uses kasniš.

This is very normal in Croatian. After ako, Croatian often uses the present tense even when English might use a future idea.

So:

• Ako kasniš, javi mi se odmah.

can mean:

• If you’re late, let me know immediately
• If you end up being late, let me know immediately

Croatian usually does not use ćeš after ako in this kind of sentence.

Javi is the imperative form, used for giving an instruction or command to one person informally.

The infinitive is:

• javiti se = to get in touch / to let someone know / to contact

The imperative forms are:

• javi se! = tell me / get in touch! (informal singular)
• javite se! = tell me / get in touch! (formal singular or plural)

So in this sentence, javi means let me know.

Se is part of the verb javiti se. This is a very common Croatian reflexive/pronominal verb.

You learn it as a whole expression:

• javiti se = to get in touch, to report back, to let someone know

So se is not optional here.
Without se, javiti usually means something more like to announce or to report something.

Compare:

• Javi mi se. = Get in touch with me. / Let me know.
• Javi mi vijest. = Tell me the news.

Mi means to me and it is the dative singular form of ja (I/me).

So:

• javi mi se = let me know literally contact yourself to me / report to me

Some useful dative pronouns:

• mi = to me
• ti = to you
• mu = to him
• joj / joj = to her
• nam = to us
• vam = to you (plural/formal)
• im = to them

Because Croatian has a normal word-order pattern for short unstressed words called clitics.

Here:

• mi = dative clitic
• se = reflexive clitic

These clitics usually go in a fixed order, and mi se is the natural order.

So:

• Javi mi se odmah. = natural
• Javi se mi odmah. = not correct in standard Croatian

This word order is something learners usually just need to get used to.

Not exactly. It is broader than call me.

Javi mi se can mean:

• let me know
• get in touch with me
• contact me
• sometimes call me, depending on context

If you specifically want call me, Croatian more directly uses:

• nazovi me

So javi mi se odmah is closer to contact me right away or let me know immediately.

Odmah means immediately / right away. Its position here is very natural.

• Javi mi se odmah. = Let me know immediately.

Croatian word order is fairly flexible, but this version sounds neutral and normal.
Putting odmah at the end gives it a straightforward, natural emphasis.

This sentence uses two different aspect choices:

• kasniti is imperfective
• javiti se is perfective

Why?

1. kasniti describes a state or situation: being late, running late.
   That naturally appears as imperfective.

2. javiti se refers to a single completed action: making contact, letting someone know.
   That naturally appears as perfective, especially in an imperative.

So the sentence combines:

• a condition/state: Ako kasniš...
• a one-time response/action: ...javi mi se odmah.

Learners often want to say this because English uses will in future-related situations, but in standard Croatian, after ako, the present tense is normally preferred:

• Ako kasniš, javi mi se odmah.

Using ako ćeš kasniti is generally not the standard choice here.
For everyday correct Croatian, use the present after ako.

Then both verbs change to the formal/plural forms:

• Ako kasnite, javite mi se odmah.

Breakdown:

• kasniš → kasnite
• javi → javite

This can mean either:

• speaking to one person formally
• speaking to several people

Yes. Croatian allows that.

You can also say:

• Javi mi se odmah ako kasniš.

This means the same thing. The difference is mostly in rhythm and emphasis, not basic meaning.

• Ako kasniš, javi mi se odmah. puts the condition first.
• Javi mi se odmah ako kasniš. starts with the instruction.

Both are natural.