Q: How can I tell the editor I will use an online language editor if the only issue with my manuscript is its language?

The language of your paper also determines its readability. If the language is unclear, then the meaning you intend to convey to readers will also be unclear. The journal may have asked you to revise the language of your paper because they may have felt that it would not have been understood by their target audience. However, there is no way to tell if the journal will accept or reject your revised manuscript now that an editor has altered the language. This is because language may not always be the only reason for rejection.

Therefore, it is not a good idea to recommend the journal not to reject your paper due to language problems. If they feel that your paper will be valuable to their readers, they will get you to work on it till they feel satisfied. It might be a better idea to request the journal editor to give you specific feedback, so that you can further improve your manuscript. 

From your question I gather that you have edited your paper, so the language should now meet the journal's publishing standards. While resubmitting it to the journal, be sure to mention that you got an editor to work on your paper. You must also carefully check all the changes made by the editor before resubmitting your revised manuscript. This is to ensure that the editor has not changed your paper's intended meaning, and has helped to bring out the impact of your work, in addition to correcting superficial grammatical errors.