Journal standards vs. personal standards
There are many different notions of quality of a paper:
- accessibility of the presentation,
- soundness of the arguments and conclusions,
- relevance of the research.
Ideally, the level of journals only differs in Point 3. Realistically, there are some differences in Point 1 as well (ironically, I find that middle-level journal score best here, but that’s a different story). However, even mega journals aspire Point 2 – and outrage ensues if they clearly fail.
I would therefore not let the level of the journal influence assessment of deficiencies with respect to soundness, i.e., if I consider a paper unsound, I recommend to reject it, no matter the level of the journal. Also, think for a second what would happen if all reviewers would recommend to accept all papers that are better than the worst paper in the journal in question: Due to the variability and laziness of reviewers (and the abundance of bad papers), the threshold would decrease perpetually. Nobody wants this.
Also, irrespective of the journal’s level, I note everything I see wrong with a paper, and let the editor decide whether this is a sufficient reason for rejection. As language problems (and some other presentation issues) can be fixed by a copy editor or similar, I would not even let them influence my main recommendation, but just recommend heavy copy-editing or similar to the editor.
Actually, it is the editor's decision to accept or reject. Give an honest recommendation according to your best judgement.
I suggest you make your best effort to help the author(s) improve the paper. Your recommendation to the editor can be whatever you think best. But don't give it a higher rating than you think it deserves (or lower, for that matter) as that will skew the information the editor has to work with. If you think it has low quality you can certainly say that to the editor.
As a reviewer, your role is to provide sufficient information to the editor that they can make a decision about how to proceed with a manuscript (accept, accept if major amendments are made, reject). I have reviewed some manuscripts, and used many more in research. From my perspective:
manuscripts should have been peer reviewed prior to submission, particularly if there are multiple authors. This solves most manuscript problems.
language should be clear. Everyone has their own writing style. I don't suggest changes to language in a manuscript unless there are major problems in my understanding of the prose, and I therefore assume that the journal readers will have the same problem. I don't edit prose, my edit is a comment about what precisely is unclear with a request to amend the text to clarify the content. I do not know if this is what you mean by "the language is poor" or whether you mean that the person does not have English as a first language and therefore their translations into English are poor. See how clarity is good?
state how the arguments are unconvincing. Also, your interpretation of how the arguments are unconvincing may not be at the same standard as those of others. Sometimes this has little impact on whether the manuscript is published.
explain exactly how the numeric results are hard to understand. You may be the only reviewer with expertise in how those numbers should be reported.