Adding Spanish translation

[navimath]

This PR translates the widget, help and error messages and adapts custom tactics to the Spanish language, see the list of (most of) translated commands:

• Help -> Ayuda
• Suppose -> Supongamos
• such that -> tal que
• Every tactic beginning by "We " has been translated to its proper Spanish conjugation, e.g,
  • We proceed -> Procedemos / Procedamos (two possible ways)
• By -> Por
• Using -> Usando
• Applied to -> Aplicado a
• Since -> Como
• Fix -> Sea

Calc has not been translated since it can be interpreted as in "Cálculo".

However, to avoid problems with the infamous translation of "and" (since in Spanish is just "y" which would clash with the variable name), I took the decision of naming it as "yy". This tool has been built for student to use Lean4 as a "personal teacher" for learning how to write proofs so they can do it by themselves on paper later. I find that the commands should resemble as much as possible to the one they will use on paper even if other Spanish native alternatives are possible (con, además, junto con, también). Another possibility would be some variant of "y_" that would enhance readibility but I find it more costly of writing.

Finally, I am afraid that this translation may be sometimes too literal and I would like the opinion of other Spanish speakers before merging, mostly in the Calc tactic with the addition of the syntax "pues" and "por", e.g:

-- In example.lean 
  Calc |u n - l| ≤ ε/2  por hN aplicado a n usando que n ≥ N
       _         < ε    pues ε > 0

I do not know if students will fine clear the difference between por (justify a calc step) and pues (using a fact), honestly I am not really sure I get it either but I saw the other translations difference between these two as well (car and puisque for French, from and since for English)

I would like your feedback on this translation before merging.