David wantsed to immerse himself in his target language, so he triesd to chat with people on language exchange apps.

Just to clarify meaning.

He thought theat serious learners would definitely be the good language partners, but things doid not work out as he thought.

Sometimes some of them of his partners keep talking to David in his mother language, and so he doesidn’t have the opportunity to practice his target languages.

SomeOther times, he feels like there ist a lack of chemistry between him and those peoplehis partners.

Sometimes heWhen he did meets someone who is fun and also willing to help, but a time difference does not helpthey are in a different time zone.

A note on repetition: I've noticed that you started the last 3 sentences off with sometimes. It would be best to avoid using the same words multiple times within the same paragraph, especially in the same place. It's not exactly wrong, just a bit noticeable and can be jarring to the reader.

Eventually, only a few serious leAfter a long search for language partners are good, he matchesd with him after he has been looking for language pseveral serious leartners for a long timethat he clicked with.

He knew if ithe is not interesting when hengaged while learnsing a language, he willould start to lose motivation very soon.

Although he feels frustrated from time to time, it is still more fun to communicate with people in his target languages than to learning the languages from textbooks independently.

Alt: to learn languages from textbooks on his own.

He tries to lower his expect lesations and to appreciate the various things he can learn by talking to people, no matter how long they candecide to keep in touch with him for.

That makes David feel much better inabout his language learning journey.

David wantsed to immerse himself in his target language, so he tries to chatted with people on language apps.

(Please read the overall feedback section at the end before you read the corrections! What I've corrected looks really harsh without context and your original text is written well.)

In this context, "Tries/tried" is superfluous in both tenses.

He thought the serious learners would definitely be the good language partners, but things do not work as he thoughts expectations were quickly tempered.

"his expectations were quickly tempered" is a phrase that has the same meaning as your original wording.

Sometimes some of them keep partners insisted on talking to David in his mother language, and he doesn’t haverobbing him of opportunity to practice his target languages.

This sentence and the next two start with the exact same word. There are stylistic reasons to do this, such as to make the rhythm of your sentences punctuate the pace your reader is reading. In this case, it isn't fitting and doesn't sound good.

The second half of the sentence was reworded to intensify David's inconvenience. Being "robbed" of opportunity is much stronger than "not having" opportunity.

SomeOther times, he feels like there is a lack of chemistry between him and those peoplelt the chemistry was lacking.

Even though "Some" isn't the same word as "Sometimes", they start with the same syllable so it still doesn't sound right to have two in a row. "Other times" meaning is almost the same as "Sometimes", but only after a first set of conditions has been established. Additionally, it links the meaning of the sentence closer to the first one in the reader's mind. The linking property of "other times" means we can make the sentence much shorter, because the context this sentence wants was already defined in the previous one.

Sometimes he meets someoWith his luck, a language partner who iwas fun and willing to help, but a time difference does not help would often be in a difficult timezone to speak at length with.

Going from "Other times" back to "Sometimes" sounds weird, so the sentence is reworded. "With his/her luck" is a construction used to mean "this is how the person's desirable outcome becomes undesirable".

Eventually, only a few serious learners aremained as good matches with him after he has been looking for language partners for a long timeafter ages of partner searching.

"remained as" is a more specific and stronger equivalent to "were" in this sentence, and "after ages" is a shortened turn of phrase that means "after a long time".

HeDavid knew if it is not interesting when he learns a lathat if he wasn't enguaged, he willould lose motivation very soon.

Because of the context of previous and following sentences, "engaged" encompasses the meaning of "interesting when he learns a language". David's name is reintroduced because I felt the pronoun was beginning to lose meaning and needed its meaning refreshed.

Although he feelslt frustrated from time to time, it iwas still more fun to communicate with people in his target languages than learning the languageslearn from conversation than learn from the textbooks independently.

Changes made to reduce length.

He tries to expect less and to appreciate the things he can learn by talking to people no matter how long they can keep in touch with himWith this in mind, he lowered his expectations and learned to appreciate the things he could take away from conversation, no matter how brief.

Changes made to reduce length. "With this in mind" ties this sentence to the observation David made in the previous sentence.

That makesde David feel much better during his language learning journey.