Before I show you some concrete recommendations from reputable bodies, I’d like to present one paragraph from the third chapter of Robert Bringhurst’s The Elements of Typographic Style to better set the stage explaining how we got to where we are with our constant interplay between roman and italic in print.
Italic and roman lived quite separate lives until the middle of the sixteenth century. Before that date, books were set in either roman or italic, but not in both. In the late Renaissance, typographers began to use the two for different features in the same book. Typically, roman was used for the main test and italic for the preface, headnotes, sidenotes and for verse or block quotations. The custom of combining italic and roman in the same line, using italic to emphasize individual words and mark specific classes of information, developed in the sixteenth century and flowered in the seventeenth. Baroque typographers liked the extra proved so useful to editors and authors that no subsequent change of typographic has ever driven it entirely away. Modulation between roman and italic is now a basic and routine typographic technique, much the same as modulation in music between major and minor keys.
Here are some of those style recommendations for modulations in the case of foreign terms.
With pen and ink
In documents written with pen and ink, you can switch into a hand that’s different from the one you’re using for the main text, such as by using an unconnected hand (“printing” in the language of schoolchildren) combined with a cursive hand (“writing” in the language of schoolchildren) for the material on which you wish to place special emphasis.
Or vice versa, as it doesn’t really matter which paired hands you choose provided they’re distinct from each other.
At your typewriter
In typewriter copy, as in non-calligraphic handwritten copy, the custom is to underline anything you would set in italic if you could. When you get your typeset copy back from the printer, it will have been converted with that understanding in mind.
On the web
In his fine book on Practical Typography, Matthew Butterick writes this in his
section on italic:
Foreign words used in English are sometimes italicized, sometimes not,
depending on how common they are. For instance, you would italicize your
bête noire and your Weltanschauung, but neither your croissant nor your
résumé. When in doubt, consult a dictionary or usage guide. Don’t
forget to type the ᴀᴄᴄᴇɴᴛᴇᴅ ᴄʜᴀʀᴀᴄᴛᴇʀꜱ correctly.
The Chicago Manual of Style makes this recommendation:
[7.49] Italics are used for isolated words and phrases in a foreign language if they are likely to be unfamiliar to readers. If a foreign word becomes familiar through repeated use throughout a work, it need be italicized only on its first occurrence. If it appears only rarely, however, italics may be retained.
That’s muddy enough, yet that is hardly all Chicago have to say about it. Their rules 6.93, 11.6, and 14.109 also touch on this matter. Their overall directions on this remain no better than fuzzy.
As is par for the course, the Associated Press Stylebook gives ungraceful directions fit only for a manual typewriter. They would see nothing but quotation marks to enclose the foreign term followed by its definition in the body of the text with no further adornment.
I would not use the AP style guide for anything unless you paid me to. A lot.
The OED always sets “mentions” in italic, such as you find in a word’s etymology section. They then follow that with the translation set in roman if needed. Here’s the one for your cloacae maximae:
Inflections: Plural cloacæ, cloacas.
Origin: A borrowing from Latin. Etymon: Latin cloāca.
Etymology: < classical Latin cloāca (also clouāca, cluāca) underground drain, sewer, in post-classical Latin also privy (from 13th cent. in British sources), cavity in certain animals (1651 in the passage translated in quot. 1653 at sense 2a) < cluere to purify (< the same Indo-European base as lutter adj.) + -āca, suffix forming nouns.
The Economist Style Guide says to put the translation in parens if need be:
foreign languages and translation
Occasionally, a foreign language may provide the mot juste. But try not
to use foreign words and phrases unless there is no English alternative,
which is unusual. So:
- a year or per year, not per annum
- a person or per person, not per caput or per capita
- beyond one’s authority, not ultra vires
(See also italics.)
translating words and phrases
If you want to translate a foreign word or phrase, even if it is the
name of a group or newspaper or party, just put it in brackets without
inverted commas, so:
- Arbeit macht frei (work makes free)
- jihad (struggle)
- Médecins Sans Frontières (Doctors Without Borders)
- Pravda (Truth)
- zapatero (shoemaker)
foreign words and phrases
should be set in italics unless they are so familiar that
they have become anglicised and so should be in roman. For
||coup d’état (but coup de
||foudre, coup de grâce)
||de facto, de jure
|bona fi de
|en masse, en route
|nom de guerre
Set in italics:
|cabinet (French type)
Remember to put appropriate accents and diacritical signs
on all foreign words in italics (and give initial capital letters
to German nouns when in italics, but not if not). Make sure
that the meaning of any foreign word you use is clear. See also
For the Latin names of animals, plants, etc, see spelling
Seeing that there are so many style standards, if it’s up to you then you should just pick whatever style makes sense to you and stick with that.
If it’s not up to you, you should just do whatever they force you to do.