If you applied the fan-translation patch (which is 95% complete as of 2024), the Gecko codes still work. The codes target memory addresses, not text strings. However, the "Gallery Unlock" code may display corrupted text – this is cosmetic only.

Are you playing on an or the Dolphin Emulator ?

: Unlock all characters and have at least one playable bodyguard at MAX friendship level (Level 5). Sengoku BASARA Wiki Farming Tips: Gold & Experience XP Farming

To unlock the Demon King, you must first complete the game with specific characters to unlock the "Honnoji" route, ultimately defeating him in his story mode.