The BE texts also use Oxford spelling, so words like “organise” are spelled with a “z”. All in all, there’s only a few words left that deviate, which leads me to consider the translation path.

To come a decision, the answers to some questions might help:

1) What is the reason for changing the BE texts rather than the AE texts? Is the majority of the texts written in AE, is it a North American publisher, is the book aimed at the North American market?

2) Is this only about spelling? As you probably know, BE and AE differ on a number of aspects other than spelling. A text that uses AE spelling, but is riddled with BE expressions (or the other way around) is likely to confuse readers. How British or American do your texts ‘sound’, spelling aside? If their tone distinctly places them on one side of the Atlantic, only changing the spelling might be less advisable and really translating them feels overly intrusive to me.

3) Could you get in touch with the authors of the texts? I would be displeased if anybody made any changes to a text of mine without notifying me prior to publication. Spelling is not a big deal and I would probably agree to minor changes, but I would want to be consulted. If you cannot reach the authors, I would base my decision on how important the linguistic form is for the texts in question. Before smoothing out the differences, you should be fairly certain that the authors did not feel strongly about using one particular variety of English.

4) How important is consistent language use anyway? If you read a text for content, you’re likely not to even notice (or at least not be distracted by) minor spelling differences. Also, juxtaposing texts in slightly different varieties of one language is common in publications of all kinds. Consistent typographic formatting does feel important to me, consistent spelling: less so. Even authors writing in the same variety usually sound different and readers can easily deal with that.

What would I do? Ask the authors. If that’s impossible, I would leave the spelling unchanged, but format all texts according to CMOS.

R. makes a good point in that British authors would be expected to write in BE and American authors in AE. Why fix something that is not broken, that said what the client (the publisher) wants can always trump what the designer/typographer thinks.

1) The majority of the texts are AE. It is a Norwegian publication, aimed at the international market.

2) One of the articles is written by a Pakistani man (using some typical BE expressions). The rest are written by Norwegian academics.

3) Yes, I can get in touch with them.

4) I’m not sure. That is my reason for asking. Most of the texts come across as fairly international, with only a few words (repeated a large number of times) in BE.

I certainly wouldn't change the spelling of a proper noun (Green Education Programme).

I think much would depend on the nature of the publication.

For example, in a literary publication where the writers words are sacrosanct or where a sense of place is important to maintain, the spelling, idioms and regional idiosyncrasies would be important to keep.

On the other hand, if the publication is, say, a collection of technical articles where clarity, brevity and uniformity are important, more editing for stylistic consistency would be warranted.

Another thing to consider would be whether or not it's a one-off publication or a periodical. In the case of the latter, matching the style of other editions of the periodical might be important, whereas that wouldn't be a concern in a stand-alone publication.

The target audience also needs to be considered. Will they be primarily British, American or an international crowd more inclined to prefer one over the other?

In any of these cases, I'd be consistent with punctuation. Swapping the position of the quote marks and periods depending on whether or not the writer is British, American, Norwegian, Kwakiutl, or whatever would simply look like a mistake, whereas maintaining the spelling of, say, color vs. colour would add to the regional flavor of the article.

Hmm - very reasonable points were made in the comments so far, but in the absence of any new input from the publishers/authors I would agree with bojev, and just keep the text as it was in BE and AE. Particularly if you are acting purely as a designor rather than an editor. Perhaps a note at the beginning of the book can explain the inconsistencies of spelling etc. in case any reader raises any questions? Good luck.

Another consideration is that an international readership is more likely to use AE.

You've noted several alternatives.

The articles written in British English might be:

1) Not modified at all;
2) Changed only in formatting to correspond to the Chicago Manual of Style;
3) Changed in spelling to be in American English spelling.
4) Translated completely into American English.

Alternative (3) is not a good alternative, simply because if you don't do (4), a reader, seeing American English spelling, will expect American English idioms, and thus possibly fail to recognize British idioms found in the text.

Alternative (2) may be acceptable; if it's a question of things like not including a space between a word and a colon following it (done in some British books) then avoiding such variations in formatting avoids a distraction, and won't confuse readers. (In fact, keeping variations in formatting risks confusing your typesetter. Of course, since these days your typesetter is likely to be, at worst, a secretary in your office, and not a Linotype operator at some remote location, this is less of a concern than it once was.)

However, I'm not absolutely sure what you mean by "formatting".

Alternative (4) probably should be out of the question. However, with one paper being in British English by a non-native English speaker, that the paper may require editing to be in correct English is a possibility I can't dismiss out-of-hand. And if you don't have a native speaker of fluent British English handy in Norway, translation to American English might be an incidental result of that process.

As you can guess, I'm somewhat inclined to alternative (1). If the alterations involved in alternative (2) are all trivial, it is the best option, not alternative (1), but I can't be certain of that.

I was wondering if the CMOS didn’t have anything to say about this issue, so I looked it up in the most recent version. Here’s what it says (7.3):

Non-US spelling. In English-language works by non-US authors that are edited and produced in the United States, editors at Chicago generally change spelling used in other English-speaking countries to American spelling (e.g., colour to color, analyse to analyze). Since consistency is more easily maintained by this practice, few authors object. In quoted material, however, spelling is left unchanged (see 13.7 [about permissible changes to quotations; R.]).

I think that spelling consistency is nice to have, but too unimportant to impose it (or it should be made clear from the outset that AE is expected and everything else will be changed to AE). In your case, I would ask the authors of the BE texts for their permission to change the spelling to AE. If there’s only one text that sounds British, I would try to live with the mismatch of idiom and spelling. Asking the author to ‘internationalise’ his English seems a bit too much to ask for. If you can’t obtain permission from all authors, I think you should call the whole thing off, typeset everything according to CMOS and know that you’ve done your best.

Normally these decisions would be made by the editor, not the designer. Your question implies you are taking on both roles, but you might consider getting a professional editor involved.

Thank you for all input. In this particular case, the editor (or more specifically, the copy editor) hadn’t done a great job. Me and the editor discussed this issue. We consulted professionals and considered your advice. In the end, we decided to keep all proper nouns and quotes as they were written in the original sources, but changed BE to AE spelling for all other instances (again, with the help of a professional), with everything formatted according to CMoS.

We would probably consider a different approach, if not for the fact that a few terms occured a lot of times in the text. “Programme”, for example, was used up to thirty times in one single page.