Should I recommend accepting a manuscript with major revisions because of a strong topic, but quite poor content?

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.

I would reverse the question:

Should you accept a weak paper written in terrible English, just because it's about a topic you like, a niche you care about or a something nobody else is writing about?

Like mentioned in some of the comments: this could actually be detrimental for the overal interest in the subtopic, and this publication might prevent more capable people from publishing the same thing (but better) because something is already out there.

In addition, the low level of English and lack of structure might also let you fill in too many blanks and give their reasoning too much value, just because you're very comfortable with the topic. Are you sure a pass by a language editing service would fix this? If it has to be rewritten by an "expert in the field" that person might as well just write the whole thing from scratch.

Finally: rejection is (most of the time) not the end of a manuscript. You can write a shorter review message with your opinion and advice and reject it. In a few months you'll see the manuscript being published in another journal, either re-written so it got accepted in a decent journal, or published in a journal with much lower standards.