Here are the steps to enable "Samurai." mode

• Go to either the main menu or story mode, personally I recomend playing 4-2 (aka entering the level and exiting out) and then talk to the janitor since he will have a very long text, long enough for the next step.
  
  
• Start typing Samurai. with the Capital S (Use left shift for this part, using caps lock or right shift wont work) and the ending dot.
  If these 2 steps were done correctly, the game will go back to the main menu or the last played level, and "Samurai." mode will be enabled...

When enabling "Samurai." mode, ALL text in the game will be replaced by the word "Samurai.". Side note, the text in for example images won't change. Only the ones that change with the Pixel/Vector font option will change. Here's some screenshots of how the game would look like:

If you want to stop seeing "Samurai." everywhere, just type "Samurai." again and all of the text will turn back to normal. You can also just close and re-open the game.

Samurai.