Should I pay for a certificate showing the quality of my manuscript before I submit it to a journal?

No (reputable) journal is going to give any weight to such a certificate when assessing a paper. If the editor/reviewers aren't satisfied with the quality of the English, they will reject the paper.

Thus, paying for a certificate for its own sake is a waste of money. Of course, if the certificate is issued in conjunction with editing services, there may (or may not!) be value in the overall package.

Absolutely no!

I believe as you are preparing a manuscript, you have a good knowledge of English. Technical writing is different from normal writing and you will 'feel' it by reading more papers. The more you read, the more your writing skills will evolve. Take the help of your seniors, colleagues, supervisor. But paying just for English is a waste of money.

Remember, the editor and reviewers will scrutinize your manuscript primarily on the basis of science, not language. They will comment on the language if it's too bad and impossible to understand.

As said by the other members, forget about the certificate. If the language is particularly bad in terms of language you may get one of the following:

• Desk reject
• Reviewers refuse to review until the language is fixed (I did this)
• Reviewers complain about the language

Unless you got one of these, even the editing service is not warranted. Even in one of these cases, you are not rejected permanently. You can fix and resend. Thus, do as best as you can, send the paper, read the reply, and act accordingly. Do not waste money on a service that you may not need.