lingvaĵo

Decode it before you translate it. Lingvo at rank 89 is a language, one of the first hundred words you met. -Aĵ- makes a concrete instance or product of the thing: manĝaĵo is food, a made thing you eat; novaĵo is a piece of news; bovaĵo is beef, the substance an ox yields. So a lingvaĵo is a particular piece or manner of language rather than the system as a whole. Language in the mass is lingvo; language as somebody actually uses it is lingvaĵo.

Careful with the word idiom. Dictionaries often gloss lingvaĵo that way, and half of what English means by idiom is right: the idiom of the Authorised Version, the idiom of the courtroom, a group's characteristic way of speaking. The other half is wrong. A fixed expression whose meaning you cannot work out from its parts, kick the bucket, is not a lingvaĵo but an idiotismo or a frazeologiaĵo. If you can say the ... of ... in front of it, lingvaĵo fits; if you mean one set phrase, it does not.

Where it sits among the neighbours. A dialekto, rank 2,275, is a regional variety with its own grammar. A frazo, rank 859, is a single sentence. Lingvaĵo is the register in between: legal lingvaĵo, children's lingvaĵo, the lingvaĵo of a translation that reads stiffly. For Esperanto itself the word does a lot of quiet work, because the movement argues constantly about whose lingvaĵo is good, and the article on spoken Esperanto is largely a description of one.