Which aspect of vocabulary is essential when assessing language proficiency?

Understanding idiomatic language is crucial when assessing language proficiency because idioms often convey meanings that are not directly related to the individual words within them. Proficiency in a language includes the ability to grasp these non-literal expressions, which are commonly used in everyday conversation. Mastery of idiomatic phrases indicates not only a higher level of vocabulary knowledge but also cultural awareness and the ability to engage in nuanced communication.

For instance, phrases like "it's raining cats and dogs" or "kick the bucket" cannot be understood through the literal meanings of the words alone. Therefore, a strong grasp of idiomatic language is a sign of a deeper understanding of the language as it reflects the speaker's ability to navigate social situations and comprehend native speakers effectively, which are key indicators of language proficiency.