で日記を付けてきたの

have been keeping a diary in

→ で

phrase Rare #41,229

have been keeping a diary in で marks the language used. 日記を付けてきた means having continued to keep a diary up to the present, and sentence-final の adds an explanatory or emphasizing tone.

JA: 毎晩日本語で日記を付けてきたの。

EN: I've been keeping a diary in Japanese every night.

ここ数年間英語で日記を付けてきたの。

I've been keeping a diary in English for the past few years.

去年から日本語で日記を付けてきたの。

I've been keeping a diary in Japanese since last year.

ここ三年間フランス語で日記を付けてきたの。

I've been keeping a journal in French for the past three years.

数年前から日本語で日記を付けてきたの。

I've been keeping a diary in Japanese for the past few years.