add spanish translations

[newmanls]

I added the translations for Spanish, following the instructions in your contributing guidelines.

I have no experience working with Flutter nor android apps in general, so if there's any problem please let me know how can I help.

Also, thank you so much for your amazing app!

[maelchiotti]

Hi @newmanls! I tried the app with your translation and it worked perfectly.

I updated CONTRIBUTING.md on the dev branch to include 2 new steps about editing the README for your new language. Could you complete them? I will then merge your PR.

Thank you for your interest and your time!