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.