mainiti nihongo de hanasite iru uti ni, dandan zisin ga demasu.

In this sentence, 〜うちに means something like:

• while doing ...
• in the course of ...
• as one keeps doing ...

So 話しているうちに means:

• while speaking
• as I keep speaking
• in the process of speaking

The idea is that a change happens naturally over time during that activity. Here, the change is that confidence gradually appears.

So the sentence has the feeling of:

• By speaking Japanese every day, I gradually become more confident.

Both can exist, but 話しているうちに emphasizes the ongoing action more clearly.

• 話す = to speak
• 話している = be speaking / keep speaking

With 〜ているうちに, the speaker focuses on what happens during the continuing activity.

So:

• 話しているうちに = while I’m speaking / as I continue speaking

This sounds very natural when describing gradual change that happens through repeated experience.

Here, で marks the language used.

So:

• 日本語で話す = speak in Japanese

This is the same pattern as:

• 英語で書く = write in English
• フランス語で説明する = explain in French

So 日本語で does not mean location here. It means using Japanese as the language/tool.

毎日 often works as an adverbial time expression, so it does not need a particle.

• 毎日日本語で話す = speak Japanese every day

This is very common with time words such as:

• 今日 = today
• 昨日 = yesterday
• 毎日 = every day
• 毎週 = every week

Sometimes time expressions can take particles in other contexts, but here 毎日 without a particle is the normal, natural choice.

In Japanese, 自信が出る is a very common expression. Literally, it is something like:

• confidence comes out
• confidence appears

But naturally in English it means:

• to gain confidence
• to become confident
• confidence grows

So:

• 自信が出ます = confidence comes / I gain confidence

This is one of those expressions that does not translate word-for-word very neatly, but it is completely natural in Japanese.

Because 出る is an intransitive verb, the thing that appears or comes out is marked with が.

So:

• 自信が出る = confidence appears / confidence comes

Compare:

• 勇気が出る = courage comes out / I feel courage
• やる気が出る = motivation appears / I feel motivated

The idea is not that someone is actively producing confidence with a direct object. Instead, confidence arises, so が is used.

だんだん means:

• gradually
• little by little
• more and more

It shows that the change is not sudden. Confidence builds over time.

So:

• だんだん自信が出ます = I gradually gain confidence

Without だんだん, the sentence would still make sense, but this word makes the process of slow change much clearer.

Japanese 〜ます forms can cover both present and future depending on context. Here, 出ます is best understood as a general result or habitual truth:

• If I speak Japanese every day, I gradually gain confidence
• By speaking Japanese every day, I gradually become more confident

So it is not really about one single future event. It describes what happens in general.

Japanese often leaves the subject unstated when it is obvious from context.

In English, we usually need I:

• I gradually gain confidence

In Japanese, if the speaker is talking about their own experience, it is very natural to omit 私.

So the sentence does not explicitly say who is speaking, but in context it is often understood as:

• I
• or sometimes one/you in a general sense

Yes. Both are reasonable, but they highlight slightly different things.

• while speaking Japanese every day stays closer to 話しているうちに
• by speaking Japanese every day sounds more natural in English when expressing the result

The Japanese has the nuance that confidence develops in the course of doing it repeatedly, not instantly.

So good translations include:

• While speaking Japanese every day, I gradually gain confidence.
• By speaking Japanese every day, I gradually become more confident.
• As I keep speaking Japanese every day, I gradually gain confidence.

Yes, うちに can sometimes mean while and sometimes has a before it changes nuance.

For example:

• 温かいうちに食べてください。
  • Please eat it while it’s still warm.
  • This implies before it gets cold.

But in your sentence:

• 話しているうちに

it does not mean before speaking ends in a strong sense. Instead, it means:

• in the course of speaking
• as I continue speaking

So this is the during the process use of うちに.

The comma simply separates the long introductory part from the main result:

• 毎日日本語で話しているうちに、
• だんだん自信が出ます。

It makes the sentence easier to read. Japanese commas are often used more flexibly than English commas, so this is mainly a natural written pause rather than a strict grammar rule.

Yes, and it would sound natural too, but the nuance is a little different.

• 自信がつく = to gain confidence, to have confidence develop
• 自信が出る = confidence comes out, confidence emerges

Both can work in many situations. In this sentence:

• 自信が出ます gives the feeling that confidence begins to appear naturally as a result of practice.
• 自信がつきます may sound a bit more like confidence becomes established.

So both are possible, but 自信が出ます is perfectly natural and expressive here.