Answer:
In the sentence, the part "who understand languages very well" is an adjective clause. Its function is to modify the pronoun "those".
Step-by-step explanation:
Just like an adjective does, an adjective (or adjectival) clause has the purpose of modifying a noun or a pronoun in the sentence, offering more information about it. Adjective clauses have a subject and a verb, and they are often introduced by relative pronouns such as "who", "which", or "that". Example: The person who called seemed to be in danger.
In the sentence we are studying here, the adjective clause is "who understand languages very well," introduced by the relative pronoun "who". Its purpose is to modify the pronoun "those".