kanozyo ha mainiti nihongo de nikki wo kaite iru kara, nihongo ga dondon zyouzu ni naru wake desu.

彼女は marks her as the main topic of the sentence: as for her...

Later, 日本語が marks Japanese ability as the thing that is changing or being described. With 上手になる (to become good/skilled), the thing that becomes good is commonly marked with が.

So the structure is roughly:

• 彼女は = as for her
• 日本語が上手になる = her Japanese becomes good / she becomes good at Japanese

In other words, は sets the topic, while が marks what is specifically doing or receiving the description in that part of the sentence.

Here, で shows the means or medium used to do something.

So 日本語で日記を書く means to write a diary in Japanese.

Some common uses of で are:

• place where an action happens: 学校で勉強する
• tool/means: ペンで書く
• language/medium: 日本語で話す, 英語で書く

So in this sentence, 日本語で does not mean at Japanese or with Japanese literally. It means using Japanese.

Because 日記 is the direct object of 書く.

• 日記を書く = to write a diary / keep a diary

The particle を marks the thing being written.

So:

• 日記を読む = read a diary
• 日記を書く = write a diary

Even though in natural English we often say keep a diary, Japanese uses write a diary.

Not necessarily. 〜ている has a few common uses, and here it shows a habitual/ongoing repeated action.

Because the sentence also has 毎日 (every day), 書いている means something like:

• she writes a diary every day
• she has the habit of writing a diary every day
• she is regularly writing a diary every day

So it is not just a snapshot of what she is doing at this exact moment. It describes an ongoing habit.

毎日 is a time expression, and time expressions in Japanese are often placed near the beginning of the sentence, before the main action.

So this order is very natural:

• 彼女は毎日日本語で日記を書いている

It sets the time frame first, then tells you what she does.

Japanese word order is flexible, but this order sounds organized and natural: topic → time → method/language → object → verb

から means because or since and gives the reason for what comes after it.

The structure is:

• 彼女は毎日日本語で日記を書いているから = because she writes a diary in Japanese every day

• 日本語がどんどん上手になるわけです = that is why her Japanese steadily gets better

So から connects the cause and the result.

In this sentence, it has a straightforward explanatory meaning: because X, naturally Y.

どんどん is an adverb that shows something is progressing quickly, steadily, or more and more.

Here, 日本語がどんどん上手になる means:

• her Japanese keeps getting better
• her Japanese improves more and more
• her Japanese gets better rapidly/steadily

It gives a feeling of visible progress.

For example:

• どんどん暑くなる = it gets hotter and hotter
• 日本語がどんどん分かるようになる = you understand Japanese more and more

上手だ means is skillful / is good at.
上手になる means becomes skillful / gets good at.

This sentence talks about change over time, not just a fixed state. That is why なる is used.

Also, 上手 is a na-adjective / adjectival noun, so before なる it takes に:

• 上手だ = is good at it
• 上手になる = become good at it

This pattern is very common:

• 元気になる = become healthy / energetic
• 静かになる = become quiet
• 好きになる = come to like

わけです is an explanatory expression. In this sentence, it gives the feeling of:

• that is why...
• so naturally...
• which explains why...
• it follows that...

So 日本語がどんどん上手になるわけです is not just a plain statement of fact. It sounds like the speaker is explaining a natural conclusion based on what came before.

Compare:

• 日本語が上手になる。 = Her Japanese improves.
• 日本語が上手になるわけです。 = That is why her Japanese improves / So it makes sense that her Japanese improves.

It adds an explanatory tone.

Because the two 日本語 phrases do different jobs:

• 日本語で = in Japanese; this tells you the language she uses to write the diary
• 日本語が = Japanese as the thing that improves; this refers to her Japanese ability

So the repetition is natural and not awkward in Japanese.

In English, we might try to avoid repeating Japanese, but Japanese often repeats words when each occurrence has a different grammatical role.

It can mean either, depending on context. But in this sentence, because of 毎日 and 書いている, it clearly means keeping a diary regularly.

So here the idea is not she writes one diary. It is:

• she writes diary entries every day
• she keeps a diary in Japanese

The context makes it a habitual action.

Yes. The pattern is very productive:

• 毎日英語で日記を書いている
• 毎日中国語で日記を書いている
• 毎日韓国語で日記を書いている

And then you can describe improvement in that language:

• 英語がどんどん上手になる
• 中国語がだんだん上手になる

So the sentence is a useful model for talking about language practice and improvement.