Traditionally, there are no diacritical marks in English. I say traditionally, because if you look at typewriters or even modern computer keyboards, there are no characters with diacritical marks.
Likewise, no U.S. newspapers published diacritical marks until recently and major news services have not transmitted information with diacritical marks.
With computers, however, it is possible to generate such characters. Even with the the standared U.S. keyboard layout, one can install the U.S. International Keyboard driver, so you can write words like Führer, which, by the way is German for "leader" and not English, so that doesn't count as to the question that was asked.
Because of computerized typesetting, some newspapers now are generating diacritical marks for foreign names or words that have been imported from other languages. Likewise, some dictionaries now accept spellings some words, e.g. "cliché" or résumé.
(Incidentally, the spell checker integral to Mozilla Firefox accepts the first but not the second)
Americans, however, are not accustomed to these words and unless somebody is competent in the language of the imported word, they are likely to come up with something comical.