Technically speaking the phrase argued about above (French?/Latin?), could be viewed as both. Romance languages (which includes French) were partially (some say heavily) derived from Vulgar Latin (that what was spoke by t' plebs) as opposed to Classical Latin (spoken by the upper crusties). Of course, other languages from the East were incorporated too. I declare a draw. Deuce !
No, it really couldn't. Do you really think Julius Caesar would have understood 'Honi soit ...'? I think not!
Do you really think Edward III could have coined the phrase had the language he was using not been derived from Latin ?
Like the other Romance languages French was, of course, over centuries, derived from Latin. But it differs significantly from the parent language, like Spanish, Portuguese, French and Italian have some similar words but are a long way from being 'almost identical'. I am fluent in (written) French, but can recognise only a few words in the other languages. So no, Eddy III probably could not. Do you believe the Royals write their own stuff? They have minions to do that.