One of the hardest things about creating a realistic, naturalistic language of your own is capturing some of the ways real-world languages create meaning. Some of those ways are less than obvious, as anyone who’s tried to translate metaphors well knows. And idioms might be the worst of all.
Definitions
Idioms, in essence, are phrases with meanings that are not obvious, that can’t be determined by looking at their constituent parts. They’re figures of speech. They often grow out of slang or colloquialisms, they may be highly dialectal, and many are simply untranslatable. An idiom’s meaning will often be figurative, not literal (though this isn’t always the case), and there can often be significant cultural associations involved.
Now, idioms aren’t the same thing as metaphors and similes. Those are comparisons. Equations, if you will. An idiom, on the other hand, is simply a fixed phrase with a specific meaning, one that substitutes for the “real thing”. And they’re also not necessarily euphemisms, because there doesn’t have to be any hiding involved. Everything can be out in the open, but it may be harder to put together, especially if you’re not a native speaker.
English, of course, is full of idioms. We talk about jokes or lies as “pulling one’s leg”, for instance, and a sad person might be described as “feeling blue”, a familiar one as “ringing a bell”. Neither of these phrases makes any literal sense, as (unless you have synaesthesia) you can’t really “feel” blue. Some others, like “knock on wood” and “cross my heart” may have grown out of actual practices, but they’ve since become idiomatic.
Translations
The problem with idioms is that, because they’re so very culture-specific, they’re exceedingly difficult to translate. In some cases, they may even be impossible to render into another language while still preserving some semblance of meaning. Literal, word-by-word translation fails utterly, because of the figurative nature of the idiom; it may be a phrase made up of words, but it’s an indivisible unit.
Many languages, especially those closely related, may have idioms with similar meanings, which helps. But a conlang, by definition, doesn’t have that luxury. Again, literal translations won’t work, so what do we do?
For auxiliary languages, the best option might be to ignore idioms altogether. The whole point of an auxlang is to foster communication, and figures of speech actively work against clarity. So, we can just distill English (or whatever) idioms to their core meaning, and translate that instead. Simple, right? (Well, wrong, but it’s close enough for now.)
More artistic conlangs are worse off when they need to translate idioms. When you’re first starting with a new language, the “distill” approach might be your best bet, but as your creation gets more involved, more complex, you’re going to have to come up with something better.
Creation
Eventually, you may need to start creating your own idioms. Either you want to use them in your conlang, or you’ve come to the conclusion that the best method of translating out of English (or your native tongue) is to swap like for like. Here’s one of the more creative areas of language construction, which naturally means that it’s one of the hardest to generalize.
Idioms are figures of speech. If you’re making a conlang that has any sort of culture behind it, then you’ll need to think about what figures into that culture. History, art, religion, science, politics, mythology, and almost anything else can come into play here. For example, a lot of our English idioms derive from medieval or early modern Christian beliefs (“the devil’s in the details”) or culture (“the proof is in the pudding”), but the past few decades have brought an increasing number of modern creations referring to technology (“information superhighway”; fortunately, that one finally died out) or media (“Netflix and chill”).
For a conlang, then, idioms require a cultural backdrop. It doesn’t have to be perfect, and you only have to go into as much detail as the work requires, but if you want to make something natural-looking, you might need to do a lot more thinking than you originally anticipated. Borrowing (which you might think is the easiest way to go) doesn’t really work, though you might be able to get away with importing a few English modernisms into a near-future conlang—you could always blame the Internet. Everybody else does.
Finally, if you want to make idioms for aliens, well, have fun with that. Seriously, those will require even more careful thought, and you’ll likely end up with a lot of phrases that simply do not translate at all. Come to think of it, that may be a good thing…
Conclusion
I know there’s not a lot here. As I said before, idioms, being figurative and not easily “buildable” in the same sense as the rest of a language, don’t lend themselves well to generalizations. We use them all the time, but it turns out to be really hard to define them precisely enough for cross-language purposes. They’re one of the hardest parts of a language to learn, because they don’t fit in, and that also makes them nearly (if not entirely) impossible to translate. Not really a winning combination for artificial languages. Hence, idiom creation is an art, not a science. The best advice I can think of? Follow your heart. (Oh, and there’s an idiom for you. What a way to end a post, right?)