I don't know; maybe some historians now translate it literally because they can no longer count on students knowing what it means, but I think most people would find 'Charles the Great' overliteral and unidiomatic.

The examples you cite where it appears only as an explanation, a translation, of the normal name, in commas or parentheses, don't seem to me to count as independent uses. I have the feeling that if someone just referred to Charles the Great without saying who that was, people would be puzzled and would not know that it was Charlemagne.

To me this is another instance where LEO really needs a usage marking that's less rare than rare, but that still gives non-natives a clue that, even if they encounter the term passively, they should be cautious about using it actively.

I agree with #1 entirely.

Charlemagne, also called Charles I, byname Charles the Great, French Charles le Grand, Latin Carolus Magnus, German Karl der Grosse (born April 2, 747?—died January 28, 814, Aachen, Austrasia [now in Germany]), king of the Franks (768–814), king of the Lombards (774–814), and emperor (800–814).
http://www.britannica.com/EBchecked/topic/106546

#1: I don't know; maybe some historians now translate it literally because they can no longer count on students knowing what it means

Zweifellos ist "Charles the Great" weit weniger gebräuchlich als "Charlemagne". Zur relativen Verwendungshäufigkeit beider engl. Bezeichnungen vgl. Googlefight (global), bzw. für UK (jeweils mit Kontrollwort "emperor").

Die Vermutung, es handle sich um eine jüngere auf Unbildung beruhende Prägung, geht aber eindeutig fehl. Im Gegenteil, ich habe den Eindruck, dass "Charles the Great" in älterer Literatur deutlich präsenter ist ('independent uses') als in aktueller. Im historischen Fachkontext war und ist die Bezeichnung jedenfalls absolut nicht unüblich - vgl. z.B. die Zigtausend Fundstellen bei Google Books.

#1: I think most people would find 'Charles the Great' overliteral and unidiomatic. (...) I have the feeling that if someone just referred to Charles the Great without saying who that was, people would be puzzled and would not know that it was Charlemagne.

Für den akademischen Kontext kann ich diese Einschätzungen nicht teilen. Gegen historische Unbildung ist ein Wörterbuch im Übrigen machtlos.

(Typing in Charles the Great on http://oxforddictionaries.com/ automatically redirects you to Charlemagne.)

http://oxforddictionaries.com/definition/Char...

Charlemagne

(742–814), king of the Franks 768–814 and Holy Roman emperor (as Charles I) 800–14; Latin name Carolus Magnus; known as Charles the Great. As the first Holy Roman emperor Charlemagne promoted the arts and education, and his court became the cultural centre of the Carolingian Renaissance, the influence of which outlasted his empire.

Supported

For me, in my general reading it has been rarer than rare, and if I saw it I would think, who is that? Or assume he's one of the many English monarchs named Charles. ;) Then I would look it up and think, do they actually say that? But it looks like some people do, or at least it is documented in some trusted sources. As I have argued similarly elsewhere in these forums, odd variants are needed so that people who encounter them can find out what they mean.

And isn't that what a dictionary is for? Wouldn't it be worse if someone typed in "Charles the Great," found nothing, and therefore assumed it must be so obviously a cognate that it's not needed in the dictionary? I think putting Charlemagne first and Charles the Great second and marking it [hist.] or [rare] or [also:] (as suggested in #0) gives the hint.

The catch is just that (also:) is also used for variants that are more or less equally common or acceptable, like, say, a compound with or without a hyphen. So seeing only that, a German student unfamiliar with actual usage in English might easily be tempted to just go ahead and use Charles the Great because it looks so similar to the German, such an obvious cognate. That's why I wish it could at least say something like (uncommonly also:), (occasionally also:), (less commonly also:) ...

And since so far all the English speakers who have responded have said we would be puzzled if we came across a Charles the Great in an English text with no further explanation, I don't think being puzzled by that unfamiliar version is a sign of lack of historical education. There have been quite a few kings whose name translates as Charles in English just in England, Scotland, France, Spain, etc., and we typically don't learn all the rulers of other countries in Europe, so for all we know it could be anyone regarded as great in some other country. How would English readers be expected to know that this unfamiliar Charles was intended to be Charlemagne? Unless you added an explanation such as 'Charles the Great (the literal translation of the name Charlemagne).'

I'm not saying don't add it, but to me the point of adding it would be to warn German speakers to avoid using it, at least without also using the usual name. So I just think it needs more in the way of a usage warning than just (also:), because it's essentially a semi-false friend.

hm, ein "less commonly also" anstelle des im OP vorgeschlagenen "also" hielte ich für eine gute Lösung.