Should I recommend accepting a manuscript with major revisions because of a strong topic, but quite poor content? I have been given my first manuscript to review for a journal. The topic is a very good one, it fills a research gap nicely and is an area that I specialise in. However, the execution of the paper is lacking. I am about halfway through making comments on it. The English is consistently poor in both spelling and grammar, and the structure is dire, which makes it hard to follow the main arguments. I am aware that bad English is no reason to reject a paper, but the English in this paper is very confused in many places, sometimes multiple times per paragraph, and it's a trial to follow.

I believe that the paper could only be salvageable after a rewrite, but by a third party. I don't believe the author's English is good enough to bring it up to standard. I do not care about being 'kind' to the author, but the topic is so good that I would love to see it go through, but I am torn between accepting it with majors or recommending rejection.  My PhD supervisor has advised me to recommend rejection, but I am still unsure which way to go. Should I recommend acceptance after major revisions in the hope that it will improve based on my [now extensive] advice or just recommend rejection?
Jeff Offutt has a very insightful scheme for calculating the final recommendation based on technical, presentation, and omission grounds (source):



The fields in the matrix correspond to different types of problems:


  Technical Problems
  Minor: Mistakes in background, related work; 
  Moderate: Does not affect the key results;
  Major: Changes the key results;
  Critical: Negates the key results.
  
  Presentation Problems
  Minor: Typos, spelling, grammar; 
  Moderate: Make understanding the paper harder  (organization, notation, repeated grammar);
  Major: Prevent understanding of part of the paper;
  Critical: Prevent understanding or evaluating a key result;
  
  Problems of Omission
  Minor: Omitted background, related work;
  Moderate: Not part of the key results;
  Major: Missing in the key results (proof or experiment, 
  lack of control in experiment);
  Critical: Must be in the paper to evaluate the result 
  (experimental study, etc) or not enough results.


The source also provides additional information and context.

Based on this scheme, I think the appropriate recommendation in your case would be a major revision.