Even if with grammar/spelling errors or hard to understand, good posts are still good in that they express some kind of unique idea or concept that is also interesting.
Except if their English writing skill is too bad, and their posts are too English-broken, otherwise posters don't need to write perfect sentences for their posts to express their opinion so that readers can understand their messages.
They can practice and improve their written English with time, and there are also local boards for discussions.
When trying to write quality posts, a lot of people act as though they're writing a book report for school: putting facts that we already know into their own words. Nobody wants to read that, and you will not get merit for it. Moreover, the length of your post and the quality of your English are only minor factors. In trying to write a quality post worthy of merit, you should offer new ideas, personal experiences, or perspectives that other forum users will actually find new and interesting.
If you are fluent in any language other than English, then it is highly encouraged for you to post in your local board. These boards often have tight-knit communities which will be able to help you, and in some ways you might be at an advantage compared to English-only posters.
In the English sections, only English is allowed. It is not necessary to speak perfect English, though you should be understandable. Try your best. If you're unsure whether your English is good enough, ask in your local board or in the Beginners & Help section
A "good post" reflects an honest attempt to contribute original thought to an ongoing conversation or discussion, even if the delivery wasn't 100% perfect. This is IMO of course, and I think this standard is fairly accomplishable... its not a high standard.
Original content with own idea and efforts in your posts is important. Nobody want to read a post composed by AI and just to get post quota from AI-spammers.