Just download standalone web page and use it offline.
Mnemonic tools for offline usage:
- dice generation mnemonic code
- javascript (NIST SP 800-22 randomness tests on the fly) generation mnemonic code
- split mnemonic code using threshold secret sharing scheme
- restore mnemonic code from secret shares
- derive XPUB, XPRIV
- derive addresses for BIP44, BIP49, BIP84
- derive addresses for BIP32 custom path