Allow other mnemonic - not based on BIP39

[codereptile]

Hi! Currently the 12-word mnemonic is used as the encryption password. Why not allow the user to set a custom password? Just 20-25 symbols give roughly the same entropy, but personally might be simpler to type/remember.

[susnux]

This needs to be a change in the RFC, so moving this for discussion.

[susnux]

Otherwise it would also be great to have localized BIP39, e.g. for German users it would be easier to have words from the German BIP39 list.

[codereptile]

@susnux That's a great idea! I didn't even consider people might not know English all that much =)