Does significant editing of a paper with the possibility of minor contributions to its scientific content warrant co-authorship? The question of whether writing/editing alone merits authorship is already addressed here. This question is about writing and editing with the possibility of minor scientific contributions.

I have been asked to review a draft paper written by a colleague (a more senior PhD student in the same research group), whose native language is not English. The main purpose is to improve the style and grammar of the paper, rather than make substantial comments to the scientific content.

It is possible that I will be able to contribute some science to the paper, however not likely of the "pedagogical oversight" variety that is normally the domain of the last several authors on a ten-author paper. 

Should I approach my colleague with the request that I be placed on the author list, only if I can make a reasonable contribution to the science (by way of some substantial comments or extra data analysis)?
The short answer is: it should not. But the reality is some think it should. Since there is no law that dictates authorship the closest to an answer is to look at the Vancouver Protocol and derivatives here exemplified by the ICMJE (Internationa l Committee of Medical Journal Editors) which defines an author as follows: 


Substantial contributions to the conception or design of the work; or the acquisition, analysis, or interpretation of data for the work; AND
Drafting the work or revising it critically for important intellectual content; AND
Final approval of the version to be published; AND
Agreement to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. 


With these guidelines in mind many co-authorships would disappear but it becomes quite clear that help with language simply is not enough for co-authorship.