At least for Spanish and Catalan, telegrams and so forth lose the diacritics; then again, they’re also written in all-caps, which isn’t the normal way either. Signed languages have words for “accent”, “umlaut” and so forth, same as they have words for a, b, c… or for punctuation. They need to be able to describe the written component completely, all of its elements.
Note that someone whose written language doesn’t include a given element may not know the word for it, even if his signed language does have it: HS-educated Spaniards my mother’s age know acento grave and acento circumflejo (both used in French but not in Spanish), they do not know ampersand or apóstrofe (both used in English but not in Spanish); those my age will be the opposite. The same is true for speakers of signed languages.
He isn’t handy to ask him any more, but per my uncle the merchant captain, signal flags are international, everybody uses the same ones and anybody who’s been at sea for any length will be able to speak several languages with worse or better grammar. Note that the most important signs aren’t spelled, they get their own flag symbol.