There is a great number of languages using diacritics: https://web.library.yale.edu/cataloging/music/diacrit
For example a simple phrase in French: l’été arrive à la fin. If one does not use the diacritics (or the correct diacritics) nothing will be found.
Or in polytonic Greek: ὃν οἱ θεοὶ φιλοῦσιν ἀποθνῄσκει νέος
Some software already does that, dtSearch (desktop search tool).