Sorry to dig this one up, yet again from the dead... but history is also unfortunately perspective...in the sense that I have recently become enamored with the middle ages (and the game) and have just stumbled upon this 'mystery' myself.
So pardon my 2 cents, hopefully they will be as well received in this 'thread revival context'...
John Grant.
From purely evidentially basis - is a creation, conjecture of his own interpretation. Being published voila the Myth.
A few points I would like to make using my own inductive reasoning. Well, I am of Greek decent... from Rhodes. But I (do I??) digress...
Most all other accounts, although only written - do identify Johannis/es as being German.
To the point of the people of Constantinople not being familiar with Scottish names, ok. Maybe. Maybe not: Germanic tribes(Goths) sacked Rome in 410. There had already been mixing with the Germanic peoples before this (some(many) had in fact been enslaved through earlier Roman conquest) but also simultaneously started to work as soldiers, rose up in ranks and eventually led a mutiny led by former Roman soldier of Germanic decent - King Alaric (Visigoths)... (this is after the period Roman's started to get complacent, exporting their work and well...)... I could go on with this.. but it is safe to say that German language was far from unfamiliar nor foreign at all to Greeks either, but especially neither to Venetians nor any Papal States! There also co-existed since the year 800 the Holy Roman Empire who was very much a player (although not through as much concentrated efforts because they were more composed of Duchies and smaller Kingdoms than any one great power at that time...
Anyways, we can also more than induce, in fact (maybe?) agree as fact that they WERE familiar with the German accent(s)... also given there had already been 4 official Crusades among many many more non official ones between Kingdom's and causes (often comprised of mercenaries and 'irregulars') German's Normans, Turks, and many many other ethnicities worked for each other, forged through aliances of varying motivations and detail that also fell at times as quickly as they were made....
If he was in fact Germanic, and _was_ large, 'Grande/Grandi'Grand' could all very well have been nicknames to reinforce the fact.
Etymology of his name? Besides the fact that we can say Runciman seems to have made this up (lacking any existing evidence to support his writing) then we must also submit some facts here on the name if John/Johannis/Johannes was indeed Germanic.
Grandt. That IS a German surname. Yet, the D is silent and for all intents and purposes is pronounce the same way as Grant. (but minus a Scottish accent
)
Any Italian/latin native speaker would understandibly write 'Grandi' or even 'Grande' for a nickname for size and/or if based on his real surname (see Grandt)
They(Venetians et all circa that area) were also familiar with the language of the Franks by this point. There is another theory that needs to be included as well. Normans also were very much involved in Europe as well as Anatolia. Remember, they made up a large portion of the 4th Crusades 'Barbarians' who took part in the sacking of Constantinople - along with other Germanic mercantiles - in 1204! That was 249 years before the fall! Yeah, let's assume most were familiar with Germanic accents by 1453... the Vainguard originally comprised of Normans, but also Germanic peoples. So let's drop Runcimen's assermation that he was a Scott yeah?
Perhaps then, let us instead induce that he was maybe one Johannes Grandt, large, an engineer, expert on counter tunnelling (love that part) and of Germanic decent.
Germanic peoples ALSO had already long been in North Africa and everything in between by 1453 (Kingdom of Vandals 453-534.) who were of Germanic origin (East Germanic Tribe). They happen to have been conquered, by Byzantine Emperor Justinian I in the 6th century. So. Byzantines, or in their own context of that time - Romans as they called themselves - were familiar with the German tongues.
Maybe, simply, all accounts forgot to include the 'd'? (Grandt)
Anyway...Nicolo Barbaro's account is mostly accepted as being the closest to factual and a wonderfull read in of itself.. (regarding the fall of constantinople) the pending doom of the empire is palpatable as each day passes and the end for all looms closer...
Thanks for reading.
Again, sorry for bringing this one back from the dead. Cheers