Ryou can speak about three different languages: Japanese, English, and Arabic. He is currently learning how to speak French as a fourth language.
Kaiba knows most of the world’s modern languages and can speak them fluently. His repertoire includes: Japanese, English, Spanish, French, Italian, German, Russian, Cantonese, Mandarin, Korean, Vietnamese, and Arabic. He doesn’t like using the Arabic too much because it reminds him of Egypt.
He doesn’t like when Ryou speaks in Arabic because his accent is distinctly Egyptian, and that just reminds him of the past which he doesn’t like to remember. Ryou just likes hearing Kaiba’s voice in a different language, despite the subject matter most likely being about dry business affairs.