In my opinion, in order to learn a language well, one may need at least nine years tof study (let us assume that the learners are not living in countries where their target language speaking countriess are spoken).

Why nine years, specifically?

Here you’ve used the pronoun “one,” which is formal, but by the end of this post you’re using “you,” which is informal. Consistency is usually better, unless there’s some reason you wanted to change the tone mid-post.

No matter how hard the learners study, their learning time is still lessimited, but if they live in their target language speaking countries, they will learn very quickly because, even if they are reluctant to study, they will still have a lot of time to studylearn passively.

Studying is always active. Learning can be either active or passive.

So if you have the chance to live in a country where people speak your target language, please take all the advantages of it.

“All” adds emphasis but could be omitted.

Study hard and you will save about 60 percent of the time [it will take when you study | it would take if you studied] in your native country.

Are you saying that it would take at least 3.6 years (40% of 9 years) if you studied in a target-language country?

In my opinion, in order to learn a language well, one may need at least nine years to study (let us assume that the learners are not living in their target -language -speaking countries).

even better phrasing would be:
(assuming that the learners aren't living in a country that speaks their target language)

No matter how hard the learners study, their learning timtotal time spent with the language is still less, but (?) than if they live in their target language speaking countries,; they will learn very quickly because, even if they are reluctant to study, they will still have a lot of time to study passively.

ok this correction is kind of a mess, but i wasn't sure what you meant with "their learning time is still less." less than what? i assume you mean no matter how hard someone studies, if they live outside a place that uses their TL, they will have comparatively less time with their TL compared to someone who DOES live in a place that uses their TL. unfortunately that's a really, really long phrase so i kind of messed up your sentence with it :,( if i'm correct in guessing what you meant, the sentence afterwards (I had to break it up into 2, because with my correction, your sentence would be like a paragraph long) should say "the latter" or "those people" instead of "they" to clarify that you're talking about the people living in their TL.
i've made a bit of a run-on sentence here so if you don't understand i can clarify

and "even if they are reluctant to study" is an aside / interjected comment; you could put it hypothetically anywhere in that sentence, so it gets separated with commas.
that's something most native speakers would probably omit, but you write really formally so i'm trying to be Very Correct with my corrections lol

People around the learners speak the learners' target language, the environment surrounding the learners is full of their target language, and the learners can use their target language every day.

nothing grammatically wrong, just that the second item in that list (the environment is full of their TL) is kind of redundant, since you basically said it already with the first item (people speak the TL)
i know you're implying signs and stuff are also written in the TL, but when you say the whole environment is full of their TL, that also includes people speaking it. which is like. the bulk of language learning

i feel like a high school english teacher but your english is so good and formal :,)

So if you have the chance to live in a country where people speak your target language, please take all thefull advantages of it.

not grammatically incorrect, but "take full advantage of it" is common phrase i hear all-together like that. so it sounds more natural than "take all the advantages of it," the latter just sounds like a new recreation of the same thing. (which is probably because it is lol)

Study hard and you will save about 60 percent of the time it will take when you study in your native country.

i'm confused what this means

study my TL hard (assumingly in my native country or on my own) and i will save 60% of the time i would have spent studying in my native country/on my own? the rest of your writing was encouraging people to move to an area that uses their TL, so i'm confused why this suddenly says "study hard" and i'll save time when i study in my own country. also it's kind of confusing what the 60% is - i'll make 60% more progress in the same time if i go to a TL-speaking-country? how do i save 60% of my time, i'll spend 60% less time getting to the same level if i live in a TL-speaking country than if i just stayed in my native country? maybe im tired and cant think straight idk

i think you meant something more like "Immerse yourself and you'll make much faster progress than if you studied in your native country." ?