Diacritical marks are glyphs (marks) added to an alphabetic letters to change pronunciation, as a syllable break, and/or meaning. It is a myth to think that English does not have diacritic marks - belovèd, café, éclair, façade, naïve, rôle are just a few of the words that have them. Diacritical marks are essential in many languages - their absence or presence changes meaning - and they should always be used.
In Hungarian, tőke means 'capital', whilst töke means 'testicles'.
In Hungarian, szár means 'stem', whilst szar means 'ouch'.
In Norwegeian, se og hør means 'look and listen', whilst se øg hor means 'look and adultery'.
In German, schwül means 'humid' or 'sultry', whilst schwul means 'homosexual'.
In Spanish, año means 'year', but ano means 'anus'.
In this example from Stockholm International Airport, the second character, which should be 達, has not been able to be reproduced with the software installed and has been replaced by a generic box symbol. This has not been noticed before the sign was erected.
Always use diacritical marks - their presence or absence changes meaning. Do not design systems or input forms that cannot accept them.
There are many technical issues in reproducing scripts and glyphs in different languages *