In fiction, a character who speaks many language denotes that they are well-educated and/or well traveled. Solely the latter is usually the most interesting; an adventurer, guide, or con artist.
I read yesterday that former Atlanta Braves center fielder and recent Baseball Hall of Fame selection speaks 6 languages.
On The Big Bang Theory both Howard and Sheldon spoke a bit of multiple languages (Howard had a working knowledge of Russian (enough to qualify as an astronaut) and enough to pick up women in several other languages, while Sheldon knew a bit of several languages (and was probably pretty fluent in German, since he lived there for a while))
Another polygot athlete is Hakeem Olajuwan, who is fluent in 5 languages.
Presidential hopeful and former Secretary of Transportation Pete Buttigieg is said to speak seven languages, though he’s told reporters “it depends on what you mean by ‘speak.’”
What has changed is that mostly everyone you encounter (also) speaks English, whereby English has become the de facto standard for communication between people with different native languages. IIRC this still wasn’t the case 20-30 years ago, when you would often encounter shop and restaurant personnel in Europe even in large cities who did not speak English.
So there is no need to address a varied group of people with all relevant languages; you can simply use English as the lingua franca. Polyglots still abound but they don’t display their skill too often.
I had the same thought when I read the OP. It feels like a very American observation (wow, it’s such a distinctive trait! etc).
Both of my daughters are fluent in four languages. My older daughter has been studying a fifth for the last couple of years and can carry on rudimentary conversation, and has plans for a sixth. My younger daughter already knows which fifth language she’ll study when she starts lycee next year as well as the sixth language she wants to acquire after that.
Our country, admittedly, is a bit of an outlier — the average person speaks about 3.6 languages and less than one-sixth of the population speaks only one or two. (Graphs in this article for the curious, though the stats are a few years old now.)
When my older daughter is talking with friends, it’s very common for them to bounce around between languages from sentence to sentence, and sometimes in the middle of sentences, expressing the majority of the thought in one language but dropping a word or phrase from another language into the middle. I’ve asked her why they do this, and how they decide which language to use from moment to moment, and she can’t explain, beyond a vague instinct for which language will be best for expressing any given thought, combined with an awareness of the skill level in each language of everyone in the group to maintain accessibility.
Being a small country, we don’t make a lot of media, maybe two or three movies and TV series per year. Our highest-profile project is probably the Netflix show Capitani (a generic “criminal underbelly” type thing), which has multiple characters switching between three different languages depending on setting. And it’s not a “polyglot trick,” it’s just how things are here.
But even so, if you travel outside our borders, it’s still quite common to meet people who speak two or three languages. Those who speak only one are pretty much limited to rural areas where they have no need to interact with outsiders.
So, yeah. The subject line of this thread strikes me as odd.
Wow, so you live in Luxembourg. And there’s a television show from there. I was astonished to learn that the actress Vicky Krieps is from there. I was also interested when I learned that science fiction editor Hugo Gernsback is originally from there. Anyway, most of the people of the world are polyglots. We talk about polyglots more because most of us are polyglots.
In fiction at least there seems to be more incidents these days of it just being said that a character has started speaking in other languages than English, rather than writing the words out.
Yep. I started this thread a while back to mention the transition from our old Grand Duke to the new one, but there was a lot of curiosity about our obscure little state so the thread rapidly became “I live in Luxembourg, AMA.” I brought up the same small roster of Luxembourgish celebrities you did, in fact.
I think a is missing. 
That is a wise remark. Even among people who grow up speaking two or more languages at home as a child there is the debate about whether true bilingualism really exists. In other words: Can you speak two or more languages perfectly at the same time?
In my case, for instance, I grew up in Madrid speaking Spanish with my brother, neighbours and at school, Catalan with my father and German with my mother. Am I trilingual? Well, yes and no. I speak the three languages like a native, but not equally well, and not for all subjects.
As I went to school in Madrid, my arithmetical thinking is easier in Spanish. I used to multiply in my head in Spanish. Catalan is mostly home related, I don’t naturally think in Catalan when I think politics or history. But I do for domestic things, like food, garden… German has been my daily language since I came to Berlin in 1982: my most used language. My language for friendship, flirting, culture, daily politics etc. And then there is English and Italian and French, but let’s leave them aside as they are not really my languages.
Knowing the state of US politics I think that Pete Buttigieg is being sensible when putting his mastery of several languages into perspective. Otherwise it would be a cheaply scored point to ask him “and how do you say this-or-that in choose-one-language-he-speaks” out of the blue and make him look ridiculous. Ask me how I know this asshole move. So many words are difficult to translate without context, and people know so little about languages on average that you, i.e. Pete Buttigieg, can only lose.
So the answer to my opening question is: no, you can’t even speak one language perfectly. There is no such thing as perfect mastery of a language. Not even Shakespeare or the Queen spoke perfect English. There will always be some word, expression, rule that you don’t know. And if there weren’t there soon would be, because languages change and evolve. Something new will come that you ignore, because it is not there yet, it does not exist. And when it comes you will have to learnt it. Or not, we are all free to rant about the decadence of the language as used by the youth, or those overseas, or the lower/higher classes or whatever. Only it is not decadence. It is just normal change.
The only really decadent attitude is not learning any language because you don’t “need” it. Fortunately there are not many countries where this is the norm.
Oops, Andruw Jones.
The technical term for this is “code switching”. It’s quite common in many parts of the world. In Africa, the bouncing is usually from an African language to the official language (English or French mostly), although bouncing between African languages is also a thing.
More broadly, multilingualism is the norm in much of Africa. Most Africans will speak a regional language, which might be limited to a single town or group of villages, as well as a more widely used African language, and the official langiuage, like English.
Saying " I can speak three languages" is like an American saying " I can drive a car".
Living in / near Miami as I do we have a vast Latin American contingent. Whether born there or here, they almost all speak their heritage language, be that Spanish or Portuguese.
It is extremely common to be someplace public like a restaurant or park or mall and have a group of 2, 4, or 6 Latinos right nearby happily yakking away. If they’re elderly, it’s probably pure Spanish or Portuguese. If they’re under 60 there’s a few English words thrown in her and there. If they’re 20-something, It’s definitely Spanish or Portuguese majority, but there are a LOT of English words phrases, and even whole sentences in there too.
Fun to listen to. Wish I could participate well enough to not be a drag.
Douglas Hofstadter likes to say he speaks exactly “pi” languages.
I assumed (because this is in Cafe Society and the examples in the OP) that this thread was about the decline of polyglots in fiction. I see no such decline in real life; not even in a growingly insular USA.
I think that’s the convo the OP wanted to have.
I don’t think it’s the one he’s getting. At least not much anymore.
IMO @Wendell_Wagner, @Tusculan and @scudsucker nailed the two main aspects.
As a matter of English language literature, polyglots had big novelty value 50 -100 years ago. Now? Little to none. Hence their disappearance as a literary stock character or standard device.
As a matter of IRL practicality English is now enough of a planetary lingua franca that extensive polyglots (as opposed to “mere” bilingualism) are becoming rare. Now it’s much more a matter of “Local heritage language, plus some flavor of English.”
As an example of the above quote, my wife is fluent in spoken Cantonese but cannot read or write it.
Which is why I brought it back around to Luxembourg’s Capitani series, because the whole idea of a polyglot being a “thing” in fiction is kind of meaningless when everyone is already multilingual.