Its not fair, you can't do anything about it, dealing with journals is not easy, and it requires a lot of patience, just send your article to another journal, and hope for the best.
You don't face this problem if the changes were highlighted. Some very busy reviewers do not like to go through the whole text detecting the revisions, which is fair in a way. Also at the same time there are certain authors without clearly addressing the reviewers comments they try to evade the issue by confusing the reviewers. Then sometimes you can expect this type of responses from the reviewers.
I believe that the bad language, and the lack of clarity as well as the technical weakness of the research are the sufficient reasons to reject such manuscript
If you are speaking of top-tier journals, approval or rejection of manuscripts go through rigorous process that it is very difficult, thereafter, to ask if rejection is fair or not.
Some claim that, at times, there is an exercise of bias or discrimination. It is often forgotten that there are basic requirements for the approval of manuscripts. Failing to meet any of them, might result into rejecting its publication!
I think It is fair and must be fair for betterment of science academia, however, nowadays few cases of biasness have been observed in science such circumstances should not be entertain in research it is against the research ethics.
Yes it is fair. You have to follow the guidelines given during revisions, which include highlighting the changes made so that it will be easy and faster for the editors to track the changes suggested.