@bananacue theres no minimum length but avoid posting one word liners and then get upset at not getting merits for the said post. The minimum qualification is proper command of the English language, and along with it try and make it helpful and filled with facts so other users like reading what you have posted.
Quality does not correlated with languages or length of posts. Posters are able to receive merits from their recognised contributions by some of their good posts.
I say some because not all of good posts will be read by the others. It is the fact because of the forum has still been affected by lots of shit posts. The another reason is merit sources are unable to spend their time to find and read all of good posts in the forum, and sometimes good or un-good (not shit) posts are subjtectively judged by readers.
From this aspect, language is not the most important thing. I agree that posts written in good English will do help rising the probability to catch attention of readers, and when readers understand most part of what posters want to mention, and when readers like or agree with posters' opinion, readers will probably send merits to the posts they read and like / agree with.
Overally, good posts always have higher probability to receive merits with whatever language (English or locals) but English-written posts might have a little higher probabilty (due to higher reading-times)