i18n (app.yml): Add Spanish localization (es)

[Rikiub]

What does this PR do

Add Spanish localization (es).

Standards checklist

• [x] The PR title is descriptive.
• [x] I have read CONTRIBUTING.md
• [x] Optional: I have tested the code myself

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[Rikiub]

Thanks for doing this!

Just one error I found:)

Done! Thanks to check!

[SteveLauC]

Thanks!