The answer is acknowledgment, and it is required rather than optional
Contributors who do not meet the criteria should not be listed as authors, but they should be acknowledged, with their specific contributions described.1 PLOS ONE applies the same rule in its own words. Contributors who do not meet the criteria for authorship must be mentioned in the acknowledgments, and anyone named there must have given permission.2 It also requires that the involvement of any professional medical writer be declared.2
The practical consequence is the opposite of what many authors fear. Using a language editor is not something to conceal. It is something to record, in a named section, in a form the journal has published.
Where the line sits in practice
ICMJE's criteria draw the line at intellectual content, which is a clearer boundary than it sounds.
An editor may change word order, tense, article use, preposition choice, and sentence length. An editor may point out that a sentence has two possible readings, that a term is used before it is defined, or that a figure in the text disagrees with one in a table. Editors Canada places that work inside stylistic editing, defined as editing to clarify meaning, ensure coherence and flow, and refine the language, and inside copy editing, which covers correctness, consistency, and completeness.3
An editor may not decide what a result means, supply an interpretation the authors did not offer, add a citation the authors did not choose, or write a discussion section. Doing any of those is a substantial contribution to intellectual content, and it makes the person who did it either an unacknowledged author or a problem, depending on who finds out.
The uncomfortable middle case is a sentence whose meaning an editor cannot recover. The correct response is a query rather than a repair. Editors Canada's guidance on student texts recommends exactly that. It advises editors to depend on queries phrased to put the responsibility on the writer, and to keep the writer's own wording where a sentence must be rewritten to show the problem.4 Its instruction that editors introduce no new content is the same rule ICMJE states from the journal's end.
Why multilingual authors are held to a stricter standard in practice
Nothing in ICMJE distinguishes between native and non-native speakers of English, and no journal policy quoted here does either. The difference appears in what gets suspected rather than in what is written down.
A paper whose prose is noticeably smoother than its author's previous work invites a question that a paper with uneven prose does not. That is not a rule, and it cannot be cited, but it explains why the acknowledgment matters more than the editing does. An acknowledgment that names the language editor and describes what they did closes the question before it is asked.
Two further precautions cost nothing and settle the question in advance. Keeping the tracked-changes file shows exactly what was altered, line by line, to anyone entitled to ask. Agreeing in advance, in writing, what the editor may touch means the answer exists before anyone needs it.
What EditFast does, and what it declines
EditFast works on manuscripts by multilingual researchers at the level of language, consistency, and clarity, and returns the queries raised rather than answers to them. Where a sentence cannot be made to mean one thing without a decision about the science, we ask rather than choose. Everything a client sends is treated in confidence, including unpublished data, manuscripts under review, and correspondence with a journal.
We are content to be acknowledged, and we do not ask to be. The decision about what appears in an acknowledgment belongs to the authors and to the journal that publishes them.