This seems strange to me. Do they demand this also from native speakers, and how they distinguish between native and non-native? Normally, the editor should see from reading a few paragraphs of the paper whether the language is good enough. However, you may attach a short declaration by the native speaker that he/she read and improved the paper. Generally, it is wise to give the paper to a native speaker, even when this is not demanded - see this discussion: https://www.researchgate.net/post/Why_is_it_compulsory_to_have_native_English_reader_checking_what_we_write_for_publication_in_western_journals . See also these discussions on translation tools which may be helpful:

https://www.researchgate.net/post/Which-is-the-best-software-for-research-paper-writing-with-correct-English

https://www.researchgate.net/post/Any_dataset_for_document_level_translation

Well narrated @ Wolfgang R. Dick

I agree with you

Agreed, usually this should be possible to evaluate by taking a look at the paper. If actually a document is demanded, a short note by the proof reader or an invoice for such services (if used) should suffice. Considering that authors are often economically exploited by journals in the first place, it would be better if they agreed to provide proofreading by a native speaker for all submissions.

Dear Frank Jacob,

Thank you very much for your ideas. They are really helpful.

I really appreciate!

Best Wishes

Dear Wolfgang R. Dick,

Thank you very much for your advice, and sharing the useful links. They are really helpful.

I really appreciate your contribution!

Regards

If the journal insists, it's possible to get a digital record of the proofreading. This can be done easily using a simple tool like MS Word. However, I agree with the previous gentlemen. It seems strange for them to make such a request.

Dear G. Datt,

Thank you for your additional advice.

I really appreciate your contribution!

Best wishes