Is it ethical to reject an article after acceptance?

The process is unusual for sure, but without seeing the reviews and what exactly the editor and you wrote to each other, I can't say that it is unethical. The acceptance of the article may, for example, have been conditional on improving language issues. This may or may not also have been stated in the reviews that suggested acceptance "subject to improving the presentation" or similar wording.

In the end, what you describe is an editor deciding that this improvement in presentation or language is apparently not forthcoming, and so the conditional acceptance was turned into a rejection. I'll note that you state: "And even more so because the rejection wasn't for the quality of my paper nor the fit of my article...", but that is exactly what it sounds like: the quality of a paper is not only a function of its scientific contribution and originality, but also its organization and ability to convey its meaning accurately. I have certainly rejected papers that may have had sufficient content for publication but were written in a way that made it impossible to understand or follow the arguments.

Obviously, I cannot say whether that is what happened. We lack the information for this. But in the end, I do believe that it is upon the author to present the material in such a way that it is understandable. Papers that fail to do so, even if their content is good, should be rejected.

My take is that you cannot really know what is going on at the journal. Your role is to try and produce the best possible content for your academic article and accept the decisions of the journal.

It does sound quite unusual - usually an acceptance means that it is accepted, period. You could try going down the official complaints route but I wouldn't recommend it. Editors have a lot of power.

I would suggest finding another journal for your paper. Yes it will take some time but ultimately it will end up in a better place. Perhaps the reviews from another journal will further improve the paper.

I don't see any question of ethics.
"Is it normal?";
"Is it polite?"; "Is it necessary?"

These and the like are all very good questions. An appointed editor made an editorial decision based upon the revision process. I don't see any ethical issues