When using the French word "sans" in an English sentence, should I use italics?

In the sentence, below, I am using the French word sans to mean without. Should sans be italicized? Or, should all of "sans human civilization" be italicized?

Planet Earth sans human civilization is imagined, at least by Teasdale, to beautiful, harmonious, and ultimately superior to the human-centred world we live in now.

OR

Planet Earth sans human civilization is imagined, at least by Teasdale, to beautiful, harmonious, and ultimately superior to the human-centred world we live in now.

As a matter of style, many U.S. publishers follow the general rules given by the Chicago Manual of Style, fifteenth edition (2003) at 7.51, 7.53, and 7.54 under the heading "FOREIGN WORDS":

7.51 Italics. Italics are used for isolated words and phrases in a foreign language if they are likely to be unfamiliar to readers. [Examples omitted.]

...

7.53 Proper nouns. Foreign proper nouns are not italicized in an English context. [Examples omitted.]

7.54 Familiar foreign words. Foreign words and phrases familiar to most readers and listed in Webster are not italicized if used in an English context; they should be spelled as in Webster. ...

Guideline 7.54 is the relevant one for your question about sans—and any other arguably foreign word you might be thinking of using. If the word is in the English dictionary that you normally use (and it doesn't have to be a Merriam-Webster product, Chicago's wording notwithstanding), you may treat it as an adopted English word and thus as not requiring italics to indicate its foreignness. This certainly is the case with sans, which has been in use in English since the fourteenth century (according to MW) and appears in such memorable quotations as Jaques's speech about the seven ages of man, in Shakespeare's As You Like It:

Last scene of all,/That ends this strange eventful history,/Is second childishness and mere oblivion,/Sans teeth, sans eyes, sans taste, sans everything.

So if you're inclined to follow Chicago's lead on this question, have a dictionary handy when you prepare to use what may or may not be viewed as a non-English word.

"Sans" is a common enough word in English that I would not bother with italics. But I also think in your sentence that the word "without" scans better, and I'd use that instead of "sans" for esthetics reasons.

Given that "sans" apparently entered early enough and/or become so much part of the language that it is primarily pronounced /sænz/ and not /sɒ̃/, then it is not required that you italicise. On the other hand, if you do prefer the latter pronunciation (as I do) i.e. you are borrowing from modern French, where you're using phonemes that aren't really used except in loanwords, you probably should italicise.

"human civilization" is plainly English so should not be italicised.

I'd prefer it if you did, please, for two reasons:

• I know the word sans in French but not in English ... italicizing it in a sentence warns me that there's something unusual about the word. Normally when I read a sentence I scan the whole sentence, but italicizing a word is a cue for me to scan/read that word individually.
• It's normal to use 'punctuation' or formatting for words which have a different emphasis in a sentence: and because sans is French it has a different pronunciation, and that different pronunciation (pronunciation being like emphasis) is sufficient reason for italicising it.