Nemesis 2 on the MSX contains unlockables by inserting certain Konami game cartridges in the second cartridge slot.
Inserting Penguin Adventures in the second cartridge slot turns the normal Vic Viper spaceship into a penguin, powerups turn into fishes and blue powerups turn into birds. Inserting Knightmare 2 allows you to restore your power-ups after you loose a life. Inserting Q-Bert allows you to use cheat codes on pause screen.
Gradius 2 is a bilingual rom cartridge. As Konami is an international company with worldwide sales and roots in Japan, they needed to produce two versions of their programs: one for the Japanese market and an English version for the rest of the world. For some games Konami decided not to produce two different cartridges for different language versions of a rom game, but put both language versions on one cartridge. This was a possible because the MSX computer BIOS rom contains a region code. Using that code as identifier, the game shows either the English or Japanese version.
This can easily checked when using an MSX emulator. Run that emulator with Japanese or English system roms and the Japanese or English version of the game will automatically run.