tomodati to issyo ni nihongo de kaiwa wo suru to, totemo tanosii desu.

Yes, と already marks the person you do something with, so 友達と会話する can mean to have a conversation with a friend.

Adding 一緒に gives the sense of together and makes it feel a little fuller and more natural in many situations:

• 友達と会話する = have a conversation with a friend
• 友達と一緒に会話する = have a conversation together with a friend

So it is not wrong or strange; it just adds the idea of doing it together.

一緒 is a word meaning together or the same. In this sentence, it becomes 一緒に, which functions adverbially and means together.

So:

• 一緒 = together / same
• 一緒に = together, in a together-way

Examples:

• 一緒に行く = go together
• 一緒に勉強する = study together

Here, で marks the means, method, or language used.

So 日本語で means:

• in Japanese
• using Japanese

This is the same pattern as:

• 英語で話す = speak in English
• 箸で食べる = eat with chopsticks
• 電車で行く = go by train

In this sentence, 日本語で会話をする means to have a conversation in Japanese.

Both are possible, but they are slightly different in feel.

• 話す = to speak, to talk
• 会話をする = to have a conversation

会話をする is more explicit about it being a two-way conversation.
話す is broader and can just mean speaking.

So:

• 友達と日本語で話す = speak Japanese with a friend
• 友達と日本語で会話をする = have a conversation in Japanese with a friend

The sentence uses 会話をする to stress the idea of conversational exchange.

Yes. Both are natural.

• 会話をする
• 会話する

In modern Japanese, many noun + する expressions can appear either with or without を.

The version with を can sound a little more explicit or slightly more formal in some contexts, but both are common.

This と is not the with particle. It is a different と: the conditional と.

It often means:

• when
• if
• whenever

So:

• 会話をすると、楽しいです = when I have a conversation, it is fun

In this sentence, it gives the sense of a general result or repeated experience:

• When I speak Japanese with friends, it’s very fun
• Speaking Japanese with friends is very fun

Both patterns are possible, but they feel a little different.

1. 友達と一緒に日本語で会話をすると、とても楽しいです

   • When I have conversations in Japanese with friends, it’s very fun
   • This feels experiential: when that happens, it is fun

2. 友達と一緒に日本語で会話をするのは、とても楽しいです

   • Having conversations in Japanese with friends is very fun
   • This treats the whole action more directly as a topic

So the original sentence is not just labeling the activity as fun; it is expressing the feeling that arises when doing it.

Japanese often leaves out the subject or topic when it is clear from context.

So even without 私は, the sentence can naturally imply:

• For me, when I speak Japanese with friends, it’s very fun

If you add 私は, the sentence becomes more explicit:

• 私は友達と一緒に日本語で会話をすると、とても楽しいです。

But in normal Japanese, leaving it out is very common.

楽しい is an い-adjective meaning fun or enjoyable.
Adding です makes the sentence polite.

• 楽しい = plain
• 楽しいです = polite

So:

• とても楽しい。 = It’s very fun.
• とても楽しいです。 = It’s very fun. / It’s very enjoyable.

Important point: です here adds politeness; it does not change the basic meaning.

とても means very.

So:

• 楽しい = fun
• とても楽しい = very fun

It strengthens the adjective, just like very in English.

Other common words with a similar role are:

• すごく = really / very
• 本当に = really / truly

But とても is neutral and standard.

Not especially. It mainly shows a pause and helps readability.

So these are both fine:

• 友達と一緒に日本語で会話をすると、とても楽しいです。
• 友達と一緒に日本語で会話をするととても楽しいです。

The comma is often added after a conditional phrase like this because it makes the sentence easier to read.

Yes, it is natural.

A few other natural versions are:

• 友達と一緒に日本語で会話すると、とても楽しいです。
• 友達と日本語で話すと、とても楽しいです。
• 友達と一緒に日本語で会話するのは、とても楽しいです。

They are all similar, but the original sentence is perfectly good and clear.