Level of detail when reviewing the language of a low quality paper
From Alan Jay Smith's The Task of the Referee:
Refereeing a paper can require considerable time and effort; don't waste that effort on a detailed critique of a badly flawed paper that can never be made publishable. Finding one or more fatal and uncorrectable flaws excuses the referee from checking all subsequent details.
The trick is to determine what is indeed "fatal". If you believe that your corrections have some non-trivial chance of improving a published paper at some point (possibly in a less-competitive venue), go ahead and make them, as you are contributing something to the body of literature. Otherwise, do not waste your time.
As an addendum, you are not a copyeditor, but a scientific reviewer. Ways to deal with papers with many language/grammar issues include:
- If grammatical errors are very numerous, point out classes of errors, instead of individual ones.
- Focus on errors which actually obscure scientific meaning, and let ones which are merely poor or nonstandard style go.
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.
Your question isn't about the level of language in fact. If I understand well, your real question is "Is it worth it to take 1 hour to correct a paper, since it will certainly be rejected ?"
At this question, I will always answer yes. I am personally a non-native english speaker and I love when people correct my work. I can then improve my English, which is always good.
What do you have to lose by doing it ? Nothing. And the person will even maybe be grateful to you.