Add Spanish translation

Thanks for working in Slight Backup.
I have completed the Spanish translation.
I have added myself as translator in the Spanish file, but not in the others.
I have copied an official translation of the MIT license, taken from Wikipedia (https://es.wikipedia.org/wiki/Licencia_MIT), stating that it is an unofficial translation and has no legal value (I looked at the other translations to see how others dealed with the MIT License text, but it seems that string is not in the other translations, maybe is it a new string?).