UnEnglish is the original English translation done for the game. As you might know, the creator of the game is from Spain, and English is not his first language. That's why the quality of that initial translation is not so good, in terms of grammar and vocabulary. After the game started gaining popularity, the original translation was corrected by a native English speaker, as can be seen in the "About" section:
This game has been translated from Unenglish to English by:
The original English translation remained as UnEnglish, just for fun.
The main differences between both translations can be found at the very beggining, and I grabbed some screenshots to show them:
- Grammar errors: things which are just wrong. "Whatever please me" becomes "Whatever pleases me".
- Literal translations: my first language is also Spanish, so I can recognize some of the word-by-word translations, forming expressions that aren't grammatically wrong but they will sound really strange for almost every other English speaker. "Throw dices" is literally "Tirar los dados" in Spanish, and it feels more natural in the English correction "Roll a d20".
- Overall changes: for example, some geek references have been updated, as it can be seen in the initial conversation. "Hey, in all videogames skeletons can be killed using arrows" becomes "Hey, it works in Legend of Zelda". As I haven't played the whole game in English or UnEnglish, I don't know how often the dialogs have been updated this way, but I guess that most of the changes have been made to avoid "localized jokes" that people outside the Spain wouldn't get, or just adding some extra geek references like the Zelda example.
Conclusion: UnEnglish version should only be played out of curiosity, but it will mostly look like crappy English for the ones who can speak proper English, and more geek references means more fun, so go for the English version.