How can I respond to this reviewer's remark?
If this is the only remark of the editor, then he likely didn't even read much more than the abstract. You should and can only apologize and make a new submission later.
I've noticed as a reviewer that even in top journals blatant grammatical mistakes can already appear in the abstract and continue throughout the manuscript. Still, the editor send to reviewers and in general I will make a comment on the quality of the language and give few examples with line number.
But, if the quality obscures the meaning of sentences or induces ambiguities I stop reading the manuscript and vote for rejection, because more than 1-2 major revisions are uncommon/unwanted by reviewers investing their free time and the good journals are more and more in competition for a fast review/pulishing process.
Hiring an editor can be expensive, especially with a draft containing ambiguities etc. More than polishing up your language is not their task and job. In general, for quality control of language and comprehension of your content it is imho necessary to "pre-review" before submission to a journal/editing service, that means to ask someone in e.g. your research group or a colleague to read the draft, who was not engaged in the actual research, but has an educational background in the best case. Your supervisor/professor should normally be able to judge if the quality meets the journal standards. Then there is still the option to hire an editor before submission or stick to use more standard phrases in scientific publishing, which you can find as lists via google or borrow from published articles of the journal you want to submit to.