Bip39. Although there are actually BIP39 wordlists in languages in addition to English, it's strongly not proposed to implement non-English words for wallet backups. It might make restoring the wallet difficult or not possible. BIP39 specifies a list of 2048 words from which the mnemonic phrase is picked. Just about https://thebookmarkage.com/story18649024/bip39-wallets-https-linktr-ee-bip39-getting-my-ian-coleman-bip39-to-work