In this trope, a Real Life language is used to stand in for an imaginary language.

So you're writing a fantasy novel in English, and your characters encounter an elf who Does Not Speak Common, or who does but occasionally drops in an Elvish word or two. However, you, the author, certainly don't speak Elvish; you don't have the patience to put together an entire Elvish Conlang for your work (unlike some people); and you don't want to just clatter out a bunch of gibberish with lots of gratuitous apostrophes.

But you do happen to speak, say, Czech (or you have a friend who does). So now your elves speak Czech.

Well, they don't really speak Czech In-Universe. The Czech language doesn't exist in your universe, any more than the Czech Republic does. No Czech characters will show up in your fantasy kingdom and discover to their shock that they can converse with elves. Maybe you don't even think of the Czech words you're using as what the elves are really saying — that's just how you're representing it. And if your book should get popular enough to actually be translated into Czech, the translator will probably pick another language for the elves to be speaking.

But it looks foreign, it's ready-made, it's coherent, it's less likely to be recognized or understood by your average English-speaking reader than, say, French, and you can even make the text mean what it's actually supposed to mean (or just slip in some Easter Eggs for fun). So your elf gives your heroes a graceful bow and utters the traditional Elvish greeting, "Dobré ráno, přátelé!"

The real-world language could be a Conlang, as long as you didn't invent it yourself and it wasn't originally used for the same purpose as you're using it in your story. For example, if your elves speak Sindarin, that's just a homage to Tolkien; if your Starfish Aliens speak Sindarin, that's this trope. (And vice versa for Klingon.)

Similarly, it's this trope when a small fictional country's citizens are all shown speaking Esperanto (a frequent choice for this purpose); however, a science-fiction setting where Esperanto is used as an auxiliary language is probably not this trope, since it's probably meant to actually be Esperanto.

Another use case is where a real but sufficiently obscure language is used as Simlish — i.e., it's used because the characters have to say something, but they're not meant to be speaking any particular language or to be understood by anyone, sometimes for surreal effect. (Beware of what you have them say; there will end up being someone in your audience who does speak that language.)

In some cases, this is an Actor-Inspired Element: a character needs to speak a fictional language, and the actor happens to speak a language less familiar to the audience, so they go ahead and use their own language for this purpose.

Sometimes, the choice of language isn't arbitrary — if the culture is a Fantasy Counterpart Culture, there's a temptation to pick the language of the real culture that the fantasy culture is a counterpart to (or a culture similar to it). To be used with caution — can have Unfortunate Implications if the counterpart-hood is too stereotypical. On the other hand, sometimes an author will pick a different language for the fantasy counterpart, just to make it clear this is a fantasy counterpart and not the real-world culture; for example, if they've switched to a fictional culture to avoid giving offence.

It doesn't count if the only use of the real language is in names (or other proper nouns). Just because people from a particular fictional culture are named Freybjörn and Ragnhildur and Þór doesn't necessarily mean that their language is being represented by Icelandic; they'd have to actually be shown speaking their language and have it be represented by Icelandic text.

A type of Translation Convention. Related to As Long as It Sounds Foreign, except that here the foreign language isn't gibberish; it's a real foreign language, just not the one it's standing in for. (It could be combined with As Long as It Sounds Foreign in cases where the gibberish in question consists of bits and pieces of a different, actual language or languages, as long as it's not the language that the gibberish is claimed to be.) Sister trope to certain uses of Sdrawkcab Speech. Somewhere adjacent to Gratuitous Foreign Language.

Examples: