Once I'm studyingied English inon a penpal app called "Slowly".

Because you are talking about the past, you write the sentence in the past tense. When talking about writing on a website or application, using the word ‘on’ sounds more natural :).

Now I'm focusing ton studying Japanese.

In this case ‘on’ is used to show that you have moved onto something else (for instance another language).

But I want to write in English ion Langcorrect site, and improve my English skills.

I tried to make the sentence sound more natural :).

Maybe I don't have manyI might not have a lot of time forto writinge an English diary.

Using ‘maybe’ here sounds unnatural in this case. However, you could use “I don’t have a lot of time to write an English diary.” This would work as well/ it will also sound natural to say.

I'll try to write in English every day.

Once I'mI once studyingied English in a pen pal app called "Slowly".

Now, I'm focusing to study Japanese.

But, I want to write English in "Langcorrect site, and" to improve my English skill.

Maybe I don't have manyuch time for writing an English diary...

-> make complete sentence, connect.

... but I'll try to write English every day.

Once I'mI used to studying English inon a penpal app called "Slowly"."

"Used to" is more appropriate for talking about something you did in the past but no longer do now. When referring to apps, we use "on" instead of "in."

Now, I'm focusing ton studying Japanese.

After "focusing," we need to use "on" and then the gerund form ("studying"), not the infinitive ("to study").

But I want to write in English ion LangcCorrect site, andto improve my English skills.

We say "write in English" (not "write English"), and when referring to websites, it's common to say "on" instead of "in." "Skills" is the correct plural form here because "English" is a broad area with many different skills (reading, writing, speaking, etc.).

Maybe I don'I might not have manyuch time forto writinge an English diary.

"Much" is used with uncountable nouns (like "time"), and "maybe" is replaced with "might" for a slightly more natural tone. "For writing" becomes "to write" to keep it concise and grammatically correct.

I'll try to write in English every day.

We say "write in English" to clarify that you're writing in that language.

Maybe I don't have manyuch time for writing English diaraily.

use many for countable nouns such as books or apples (it makes sense to say "I have 3 books" or "I have 3 apples")
use much for non-countable nouns such as water or time (it doesn't make sense to say "I have 3 water" or "I have 3 time")