[l10n] Improve Spanish (es-ES) locale

[soler1212]

In response to issue #3211, this PR add (or fix) the missing translation for Spanish.

• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

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[LukasTy]

Thank you for your contribution! 🙏 🎉

[LukasTy]

Nitpick: Technically having this word start in lowercase seems a bit more appropriate, given that it is used mid-sentence.
But that's not a problem now, because those cases are only used on aria-label attributes that will not be visible, just read by screen readers. 👌