我也好久也没有练习中文。

第一天用 langcorrect

Good start! The sentence is literally what I would write as a native speaker. A small change I would make is to place a space between “用” and "langcorret", so with the space your sentence will be:

"第一天用 langcorrect"

Sorry this is a very personal habit of mine to place a space before an English word or digits. Got this habit from someone in the publishing industry and they were saying that this would look nicer in print and make it easier for the reader to process information, which I agree. It's a bit like you would place a space after English punctuations. A lot native Chinese speaker/writer actually don't care for this. Just my two cents :)

我以前用过了一个类似的 APP叫 叫 lang8。它也是一个练习语言通过记日记的一种，，那个 App 也通过记日记练习语言；但是我好久没有写了。

Nice try with few grammatical errors!
In the first part "我以前用过了一个类似的APP叫lang8"，both “过” and “了” represent past perfect tense and using both is redundant and a bit confusing to read; for this sentence, "了" sounds more natural to me thus my correction would be "我以前用过一个类似的 APP 叫 lang8"

"它也是一个练习语言通过记日记的一种"
Using “它” as a relative pronoun for THE APP sounds a bit off to me. "它" is usually used for something... with life? Like I would't say 我有一支笔，"它"很好用；Instead I would say, 我有一支笔，这支笔很好用 or simply omit the pronoun altogether 我有一支很好用的笔。Thus for your sentence, my correction would be
“那个 App 也通过记日记练习语言。” (literal translation: that app also helps you practice the language through writing journal entries)

"但是我好久没有写了" is a good one with no error. But I would use a semicolon before as a juxtaposition.

写的时候我一直需要停，深得想想我会时不时停下来，想想怎么写，然后去pleco翻译不知道的一些字。

It's generally understandable ^^
My correction: 我会时不时停下来，想想怎么写，然后去pleco翻译不知道的一些字。
First of all I'll omit "写的时候" as it is a bit repetitive since you mentioned the action of "writing this paragraph" in your last sentence.
Secondly, I know dictionary will tell you “一直” is the translation for "always", but it can be viewed as a sign for present perfect continuous tense in Chinese, which doesn't make much sense in your sentence. I changed it to “时不时”, which means every now and then.

希望我每天会写日记，中文以后会都能记日记，以后中文能进步/以后中文可以进步 ^_^

You are omitting the subject of the sentence, which is “我”.
The other nitpicking I could make is the use of “会” in both sentences. I would change it to “能” or “可以”（could）. I believe there is a subtle difference between 会 and 能, in that “能” or “可以” is often used to show personal will whereas “会” is used to indicate causality.

so my correction is:
希望我每天都能记日记，以后中文能进步/以后中文可以进步。

(I see there's another native correction putting 我 at the beginning of the sentence, which is fine as well）

我以前用过了一个类似的APP叫lang8。它也是一个练习语言通过记日记的一种网站，但是我好久没有写了。

写的时候我一直不断需要停，深得想下来想一想，然后去pleco翻译一下不知道的一些字。

我希望自己每天都会写日记，中文以后会以后中文会取得很大进步 ^_^