Well, I am not native speaker of Spanish, but sounds that "the just" as translation for "justiciero" may be correct according to this site (La Real Academia Española (RAE) - The Spanish Royal Academy):
"justiciero, ra
1. adj. Que observa y hace observar estrictamente la justicia.
2. adj. Que observa estrictamente la justicia en el castigo de los delitos."
Even at Britannica, grandfather Alfonso is called "The Just" or "Alfonso el Justiciero"
https://www.britannica.com/biography/Alfonso-XI
Avenger in Spanish is vengador. In Portuguese is vingador.
I am a native speaker of Spanish.
Just is English, and applied to a person, it means to be fair (in the Salomonic way).
The Justiciero, as you can see in your quoted definition goes beyond the quality of fairness, it covers action: the ENFORCEMENT or APPLICATION quality. Fairness in enforcing justice and/or applying punishment for crimes.
I suppose the international scholars follow The Spanish Royal Academy - "La Real Academia Española (RAE)" and its translation.
Under my pragmatic view, the best people to nominate grandfather Alfonso are the Spanish Royal Academy because it's a Spanish Academy. Sounds logic. And according to his biography sounds perfect.
Looks like the Spanish scholars and Spanish people agree too.
As we can see here at the "Real Academia De La Historia" - Royal Academy of History (Spain), where he is called "El Justiciero":
https://dbe.rah.es/biografias/6406/alfonso-xi
I speak Spanish from Spain. And for me sounds 100% correct the way the both Royal Academies do. Anyway, academically speaking, they are the responsible for it.
To Eliane Prates-Svennesen: I did not dispute the Spanish Royal Academy's definition of the word "Justiciero" all the contrary: I used it to point that it goes beyond being fair,
I agree that avenger may have the connotation of revenge. I just mentioned it because I have seen it used (an improvement from "Just").
What would you call Superman and Batman?... they don't bring criminals to justice because of revenge. That's what a Justiciero does, fairly.
Just (just fair) and Justiciero are not the same, and I'll leave it at that.
All I wanted was to point out an inaccuracy. I'll go on with my life :)
I know I had willed not to continue this discussion. but I could not help it after hearing this quote from the "Lancelot" revival on Broadway:
"Justice is not revenge"
So we continue to be limited in the English language to describe someone who enforces justice, but not for revenge.
BTW. I read that Alfonso XI being described also as 'cruel'... another hue.
Well, German, I guess you really have a "Keen" on this subject!
Do you still remember this?
And yes, I agree with: "Justice is not revenge".
"Todo parece indicar que su abuela María de Molina dejó en Alfonso XI una profunda huella. Por lo demás la Crónica de Alfonso XI nos presenta a dicho monarca como un hombre serio y apuesto, con grandes virtudes castrenses, pero a la vez muy preocupado por el cumplimiento estricto de la justicia. De ahí deriva precisamente el calificativo de justiciero con el que se le conoce a Alfonso XI. Es más, dicho Monarca se mostró sumamente generoso con aquellas personas que le fueron fieles."
in https://dbe.rah.es/biografias/6406/alfonso-xi
It's important to remember that nobody is perfect. All of us do good or bad.
The important thing is to do good, right?
Alfonso XI is not the "cruelest" grandpa I've found it. But I am not here to nominate all the Ancestors' sins. I have my own to take care of.
I suppose, some centuries ahead, some members of our future families will come back here to this discussion to speak what they think about you, Cousin German, or me, for example.
Anyway, have a nice Sunday! :-)