My interpretation of the question is as follows: you are asking if in your report you should spend time correcting all the grammar errors, spelling errors, etc., of which there are many of, especially since the paper will be rejected anyways.
First of all, the answer is completely independent of whether the paper will be rejected or not. The effort you put into a review should not depend on whether you intend to reject it or not - that defeats the entire purpose of a review.
Secondly, you claim that the paper will be rejected anyways, which implies that the errors in writing have no significant influence on the scientific content of said paper. If true, state this explicitly in your review. This ensures that no unwanted misunderstandings occur (since you shouldn't reject a paper due to language errors alone, at worst you should require a major revision prior to accepting it).
Now, should you list all/most of the errors? No. That would come off as quite condescending and probably not useful at all. Instead, you should mention what tendencies you see and use one or two examples to explain what you mean. Since there are so many mistakes that it'd take you an hour to write them all down, chances are that the authors' language skills are so bad that they need to consult a professional, potentially hire an editor to write the paper for them: mention this, again in a polite way, emphasizing that neglecting to do so is likely to harm their future papers as well.
You should also remember that if your feedback on their writing takes up more space than usual, perhaps you should consider giving them even more feedback on their actual content to balance things out, even though you've already clarified that their language errors aren't the reason for their rejection. This is because authors can sometimes be so vain that if you focus too much on their writing, they'd automatically assume that "oh yeah, they must have rejected me for my writing, look how much they focused on it", even if you explicitly state otherwise. You are not responsible for their assumptions, but a good review should be well-balanced and give off the proper signals.