Punctuation – Should There Be a Space Before Any Punctuation?

The answer, as with so many other such questions, is it depends.

• How long ago did the word enter the English vocabulary? The longer ago, the less likely it is to retain the diacritics.

• How frequently is the word used? The more frequently, the less likely it is to retain the diacritics.

• Without the diacritics, does it look identical to a different English word, especially one that's pronounced differently? If yes, it's slightly more likely to keep the diacritics.

• Is it a personal name? If yes, it will almost always keep the diacritics, at least nowadays. (Even just 20-30 years ago, personal names would be Anglicized to a much greater degree than they are now. For example, the conductor born in 1899 was Eugene Ormandy, not Jenő Ormándy, but the pianist born in 1953 is András Schiff, not Andrew Schiff.)

• Is it a proper noun that is not a personal name? Then it really depends on the first three factors above, namely age, frequency, and conflict. Plus, there are place names that are translated into totally different words — for example, the difference between München and Munich is not just the umlaut. (This occurs in other languages, too: chances are, you don't call the countries "Suomi" or "Magyarország" in your native language, unless of course your native language happens to be Finnish or Hungarian, respectively.)

• Are you writing for a publication that has a style guide about diacritics? Then follow that guide, even if it contradicts your experience or your dictionary.

The only way to settle this question is to look up each word in the dictionary (or encylopedia, if it's not the sort of proper noun that's listed in the dictionary). For the words in your example, here's what Dictionary.com had to say:

résumé; also resume, resumé
café; also cafe
naive; also naïve
Schrödinger
(no listing for Citroen or Citroën; Wikipedia only has the latter spelling)

In any case, don't get hung up on "but that's the wrong pronunciation!", and especially not on "but that's the wrong spelling!" Once a word has entered English vocabulary, it is slowly but surely assimilated, and at some point — same as with literally every other English word — its etymology becomes irrelevant. Its correct pronunciation will come from context and from the reader's knowledge, same as the rest of the words in the sentence.