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.