[quote user="Renan "C#" Sousa"]
Don't forget to add support for elvish Cwenya.
I am pretty sure that an old form of English used accents for stress marking on words, and that can still be seen as when you write words of foreign origin (such as "resumé", or "fiancé").
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Those accents are inherited from French--the accent égout isn't about stress, it's about sound. "Resume" is pronounced "ree-zoom", "resumë", if such a word existed, would be "ree-zoom-uh", and "resumé" is "re-zoom-ay".
The only thing like a diacritical to exist natively in English is a diaeresis, which just indicates that a vowel is to be pronounced separately (not silent or in a dipthong). The most famous is probably on the end of Brontë (which is supposed to be pronounced "bron-tee", not "bron-tay"). The New Yorker spells "cooperate" "coöperate", which is correct, just pretentious.