Bip39. Though you will discover BIP39 wordlists in languages in addition to English, it's strongly not advisable to implement non-English words for wallet backups. It would make restoring the wallet tough or unachievable. Entropy is a sophisticated function. Your mnemonic could possibly be insecure if this aspect is used incorrectly. https://socialrus.com/story18349119/bip39-words-https-linktr-ee-bip39-bip39-generator-an-overview