To answer the thread title, R-O-L...hang on, that's something else. (Also appropriate, but not for written formats, "K-3-A-N-G-A, but the three is silent.) Cue crickets...
Anyway, yes, the dieresis (German's umlaut, basically) in English (from the French) says "this vowel is pronounced, and unrelated to it's neighbors." Thus, the name "Noel" is pronounced as one syllable, while "Noël" is pronounced as in "nobody from Superman's family. If you read enough old books, you'll also find that most double-vowels that come in compounded (or other aggregate) words are often marked the same way for the same reason, coöperation being the biggie. So it's probably pronounced Kah-Angah, where the latter is "anger" with a dropped-r at the end (think Bostonian pronunciation).
It's just a guess, but the wider use of typed correspondence (with respect to hand-written or typeset works, where any formatting or marking is possible) has led accenting to fall out of favor. I think it's an educated guess, though, because there's the tradition (also carried over from French, and I believe it's also the Spanish rule for the tilde) that capitalized letters are accented only optionally--because under many mechanical conditions, you can't guarantee that you'll have space above the full-sized form when printing.
