In the British Commonwealth tradition, regents and vice regents (governors and lieutenant governors) are “Your Excellency”, judges are “Your Honour”, and magistrates are “Your Worship”.
I have to write a letter to an ambassador. How do I address her? “Dear Ambassador”?
“Your Excellency” is correct English usage, so if you are writing to an Ambassador in an English speaking country you are probably OK using that regardless of their nationality.
As she is a lady though there may be another female specific honourific but if so it is now out of date. If they are overseas Ambassadors you might consider using the correct usage for that country - I know there are specifics in portuguese and italian but that is as far as my language skills go.
Oh, BTW whilst you start the body of the letter with “Your Excellency” the letter itself and envelope should be addressed to, “Her Excellency Ambassador xxxxxx”. I think…
I’ve looked, and Fowler’s does not appear to have an on-line version and regret I do not have mine to hand.
So maybe it is not so clear cut - though I guess all matters of custom are, well, customary and grow organically.
On the second cite, whilst I agree that ambassadors enjoy the honorific “His/Her Excellency” when referred to in the third party, when addressing them in the first party, as in a letter or personally then surely you adopt “Your Excellency”?