Salt the speech, don't drown it

Why this rule exists

Invented words and languages are seasoning, and like salt they make everything better in small amounts and inedible in large ones. A few well-chosen coinages, a name that sounds right on the tongue, a phrase in a tongue we do not speak, can conjure a whole culture in a breath. But a page thick with apostrophes and untranslated jargon does not immerse the reader; it exiles them, forcing them to decode instead of feel. The reader does not need a full grammar to believe in a language; they need the impression of one, the sense that these words come from a real system, glimpsed at the edges rather than spread across the table.

In practice

Coin sparingly and with an ear for sound and consistency, so that names and words from one culture share a music and could plausibly belong to the same tongue. Introduce foreign terms in contexts that make their meaning clear, so the reader is never stranded, and lean on the ordinary word most of the time. Prefer one perfect invented term to five adequate ones, and read your coinages aloud, since a name a reader cannot pronounce is a name they will skim past. If you are building a fuller language, keep most of it offstage as a well from which you draw a bucket at a time.

Example

"Sha'vel'ka thu Ryn'dorae," she said,
invoking the Ael'thuun'ir by the
Vel'shantari rite of Qo'thal'zin,
beneath the Dorae'shvantul moons.
(The reader has stopped reading.)

"Sky-of-mourning," she said in the
old tongue, and named the three
moons the way her grandmother had.
One foreign word would have been
enough. It was.

When it doesn't apply

Some writers are also linguists, and for them the language may be a genuine centerpiece, worth deeper display, provided the story survives the immersion. Fully rendered speech can also mark a boundary the reader is meant to feel shut out of. But for most stories, the goal is the flavor of another tongue, not a lesson in it.