So what I have to say is too long to fit in a tweet or even a handful of tweets. I followed the link in this tweet –
https://twitter.com/kissane/status/627264725697581057
…which lead to this video by Daniel José Older, titled “Why We Don’t Italicize Spanish”, where he explains why language-switches in his books are not italicized (I’m assuming, anyway, that this applies to all languages, ie. not only is Spanish not italicised, but neither is Italian, German, etc):
I have two thoughts/observations/comments:
1) Regarding speech.
Older argues that since we don’t flag language-switching in speech, it shouldn’t be flagged in writing.
Well, I disagree that languages are not marked when language-switching in speech. All languages have, apart from different vocabulary and grammar, also different prosody: they stress words differently, and have different intonation patterns. Thus when someone speaks a non-native language “with an accent”, this refers to the stress and intonation they use. So someone speaking English with a Finnish accent is using the stress and intonation patterns of Finnish while producing English syntax and vocabulary. Languages sound different.
Therefore while a speaker may not intend to emphasise either language when language-switching in speech, both languages are nonetheless flagged.
2) Regarding writing.
I find this very interesting. I work on the links between scripts/typefaces and languages in the Early Modern period, and am fascinated by aspects of such practices which have survived into present-day use. Such as the practice of italicising foreign words, phrases and passages. What people in general will have no idea about is that this practice has its roots in the very earliest writing practices where languages were marked by different scripts. The general idea is familiar to everyone from having seen texts written in different writing systems: for instance, I’m using the Roman alphabet to write this blog post, and you can immediately distinguish it from Japanese writing:
これはローマ字で書かれていません。
What is less well known, is that it used to be common to use different scripts and typefaces to write, for instance, English and Spanish. That is to say, not different writing systems or even different alphabets – such as is the case with, say, Greek:
Αυτό δεν είναι γραμμένο με λατινικούς χαρακτήρες.
In the 16th century, Northern European vernaculars (German, Dutch, English, Swedish, etc) were usually written in a gothic cursive script, whereas Italian and Spanish used scripts based on italic characters. And the same applied in print:

Anyway, long story short, this distinction of course disappeared, although gothic typefaces and scripts survived until very recently – for instance newspaper titles are often still found in blackletter. But the practice of switching script or typeface to indicate switching language was, in part, retained.
Of course, the story is far from this simple: at the same time, practices of textual emphasis developed. These included, for instance, colour, enlarged initials (capitalisation), underlining, quotation marks – and also script- and typeface-switching. Therefore whereas you might italicise a foreign word to flag it, and you might also italicise a word to emphasise it, these are in effect two different practices – even if, from a present-day perspective, they have for the most part merged. And today you could even see the italicisation of foreign words and passages as just one of many reasons you might want to emphasise text.
And this last point appears to me to be the one Older makes in his argument against italicising Spanish in his books: the italics make it look like the text is emphasised. Whereas this is not the case. And I take it he doesn’t want to give the impression that authorial emphasis is intended. It is just language-switching, that’s all.
This is great – people rarely voice their conceptions on these matters, and I find it fascinating to find out how people see methods of textual emphasis and their uses.
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Anyway, I find these things incredibly interesting and bizarrely understudied. I have several publications on this coming out next year (gods willing), in which I hope to make my case and points better (and at more length) than I do here.
NB I purposely stayed away from discussing code-switching here. I don’t think it matters regarding the general points I make above.