Proofread entire Spanish language & correct wrong phrases

Completely revamped the Spanish localization. List of major changes:

- The plural function was incorrectly programmed, so many words were displayed grammatically incorrect in the game.
- Many misspelled words (typos) have been corrected.
- The words used have been refined to ensure that they are as understandable as possible.
- A list of Spanish language contributors has been added.