結果是，我的法語比（我）姐姐得的更好了!

那時候，我覺得法語是世界第一上最好聽的語言。

所以大學畢業以後我搬家來韓國，幾年都沒使用法語，望忘了很多。

最近我想學再一次學法語。

其實我小學的時候參加了一個法與語社團，但我那時候學法語學得不好法語。

When using 得 to set off an adverbial phrase, you need to repeat the verb if the verb is also taking on an object: https://resources.allsetlearning.com/chinese/grammar/degree_complement#Degree_Complements_with_Objects

因為我從小對外語有很多興趣所以我入中學的時候決定學法語。

It sounds awkward to me without 的時候:

那時候，我姐姐也在上中學法語課。

I think the correct way to interpret this is "My sister also took high school French," so this wording should work better.

我覺得如果我學得不好，我就可以問我姐姐可不可以幫助我。

可不可以 only makes sense to me if it's in an actual or implicit question or problem (like in "問你爸爸你可不可以去公園。").

結果是，我的法語比我姐姐得更好了!

那時候，我覺得法語是世界第一最好聽的語言。

第一 also seems like it would have to be followed by a classifier, like in 第一名. But I'm not completely sure about the grammar of this, otherwise: 最 just sounds more appropriate.

從中學到高中畢業一直學法語，本來打算去大學多學法語，當法語翻譯，但我沒做這樣。

It makes more sense to me to insert 本來 and/or 最後, to imply a change. Otherwise the logic of the statement seems to be slightly lost.

所以大學畢業以後我搬家來韓國，幾年都沒使用法語，望了很多。

This was not entirely wrong, but in writing, using 所以 at the beginning of the sentence is incorrect.

我覺得我跟中學，高中學的時候一樣會學好法語!

I might be misunderstanding the sentence or wrong about the grammar but it makes more sense to me to write 中學，高中, since "高中學" doesn't quite function as a parallel unit.