Learn French: Le chef demande si le candidat bilingue est disponible. - The boss asks if the bilingual candidate is available.

Because this is an indirect question. The boss isn't directly asking you, "Is he available?" (Est-ce qu'il est disponible ?). Instead, we are reporting what he is asking. To report a yes/no question, French uses si (if/whether) to connect the introductory verb (demande) to the reported question.

I thought le chef meant a cook. Does it mean a restaurant boss here?

Not necessarily! While le chef did give us the English word for a head cook, in French it is much broader. It simply means "the boss", "the manager", or "the leader" in almost any workplace.

Why is it le candidat bilingue instead of le bilingue candidat?

In French, most adjectives go right after the noun, especially ones that describe objective categories like nationality, color, or language skills. Since bilingue describes a language skill, it must follow the noun.

Since this is reported speech, do we need to change the tense of est disponible?

Not in this case! When the introductory verb is in the present tense (demande), the reported speech keeps the exact same tense the speaker originally used. The boss's original words were likely "Is he available?" (Est-il disponible ?), so we keep est in the present tense here.