I realized that thehis sister of him often said the four-letter words, "fuck."

for the sentence to flow more smoothly :)

I realized that thehis sister of him often saidys the four-letter words, "fuck."

Likewise, in Japanese animes, lots of characters speakuse a lot of dirty expressions in Japanese.

Yet, I don't want to hear (read) non-native Japanese speakers speak (write) in those mannerslike that.

I strongly believe those words are not properlyshould not be used by non-native speakers.

It seems as though we are pointed outfeel insulted when the dark side of our native tongue, and feel like insulting our native tongue is pointed out to us.

That is why you advise non-native speakers dto not use the four-letter word, right?

Since I started using Lang-8 13 years ago, some Japanese learners have insistently imitated anime character's' or light novel character's' dirty Japanese, but I don't think they are on the right track.

Native speakers' vocabulary available and non-The vocabulary that native speakers' use should not be the same as that for non-native speakers.

I realized that thehis sister of him often said the four-letter words, "fuck."

You would use "his" to describe this, also it's only a single word so "word" shouldn't be pluralized here.

Yet, I don't want to hear (read) non-native Japanese speakers speak (write) in thoseat manners.

"manner" used in this way is always singular.

I strongly believe those words are not used properly used by non-native speakers.

This is correct but I think this ordering sounds more natural.

It seems as though we arethis pointeds out the dvulgark side of our native tongue, and feel like insulting our native tonguethereby insults it.

This makes the subject of the sentence more clear. "Vulgar" may be a more accurate word choice if you are looking to express an indecent, coarse side of the language instead of a sinister or evil one which "dark" might imply.

Native speakers' available vocabulary available and non-native speakers' should not be the same.

This word ordering seems more natural.

I realized that thehis sister of him often saidys the four-letter words, "fuck."

I watched lots of actors and actresses saying that word in other movies too, so I guess native English speakers frequently use "fuck" in their daily everyday conversation, as a natural choice.

“As a natural choice” sounds a little awkward. I would either get rid of it or maybe say “native English speakers frequently use "fuck" in their daily everyday conversation like it’s part of their nature.” This might come off a little rude though.

Do you feel like it's a good thing?

Yet, I don't want to hear (read) non-native Japanese speakers speak (write) in those manners.

“In those manners” also is a little unnatural. “I don't want to hear non-native Japanese speakers speak (in that way/like that/that vulgar.)

It seems as though we are pointed out of the dark side of our native tongue, and it feel likes insulting to our native tongue.

Im not sure what this sentence means. I’ve corrected it grammatically, but I can’t decipher what you meant to say. I’m sorry.

Therefore, when a non-native speaker speaks dirty Japanese, I would even hate them.

When you use the word “dirty” while speaking about a language it gives the connotation that your talking about sex. I would say “vulgar Japanese” instead. This refers to “bad words/curse words.” Also “I would hate them” implies that you would hate the person speaking. I’m not sure if that is what you meant or if you meant “I would hate hearing it.”

I don't want to hear fromlisten to them.

That is why you adviseI ask if you think non-native speakers do notshould use the four-letter word, right?

Native speaker’s' vocabulary available and non-native speakers'’s vocabulary should not be the same.

I realized that thehis sister of him often said the four-letter words, "fuck."

Use possessive pronouns (my/your/his/her/our/their) when discussing relationships. While English sometimes uses the "of" form to show possession (I am Tracy, daughter of Ron), it's so formal that it's mainly used in literature.
"Fuck" is one word, so no "s" required.
Great job on "four-letter"!

It seems as though we are pointeding out the dark side of our native tongue, and feel like insulting our native tongue.

I don't fully understand this sentence. It can be interpreted a couple of ways, based on what minor change I make:

1. It seems as though we are pointing out the dark side of our native tongue and we feel like insulting our native tongue. (In this case, maybe just write "It seems as though we are pointing out the dark side of our native tongue" as it is strong enough on its own to make the point.
2. It seems as though we are pointing out the dark side of our native tongue, and it feels like we are insulting our native tongue. (In this case, clarify by saying "It seems as though we are pointing out the dark side of our native tongue and it feels insulting".)

That is why you advise non-native speakers dto not use the four-letter word, right?

You advise someone to (do something) or of (something). You need a preposition after the verb in this case.

HOWEVER, you could say 'That is why you advise non-native speakers "Do not use the four-letter word," right?' Using a quotation removes the need for a preposition.

Since I started using Lang-8, 13 years ago, some Japanese learners have insistently imitated anime character's or light novel character's dirty Japanese, but I don't think they are on the right track.

Because you have two numerals next to each other, separate with a comma to help the reader understand they are different numbers.

Native speakers' vocabulary available and non-native speakers' should not be the same.

This word should not be in there at all.