Bilingual: This is the most common and general term, meaning fluent in two languages. It describes someone who can read, write, speak, and understand both languages.
Biliterate: This term specifically refers to someone who can read and write in two languages. It emphasizes the written aspect of language skills.
Polyglot: While this refers to someone who knows multiple languages, it doesn't specifically indicate reading and writing abilities. It's more of a general term for someone who speaks multiple languages.
Ultimately, the best term to use depends on what you want to emphasize. If you're focusing on the reading and writing skills, "biliterate" is more accurate. If you're speaking more generally about language fluency, "bilingual" is the most common term.