 A polyglot, Castel spoke Portuguese, Ladino, German, French, Spanish, Italian and English with native or near-native fluency.He was an internationally known language and diction coach, writing the book "A Singer’s Manual of Spanish Lyric Diction" and translating an extensive annotated series of librettos of French, German and Italian operas that show the pronunciation of every word in the operas' original languages using the International Phonetic Alphabet.These libretti "are used by singers, teachers and conservatories throughout North America and Europe."