There is not a one-to-one correspondence between upper and lower case letters.
Turkish is a good example of this. In Turkish, the letter I/i has a dotted-upper-case form (İ) and a dotless-lower-case form (ı).
https://en.wikipedia.org/wiki/Dotted_and_dotless_I
This means that you cannot correctly convert between upper-case and lower-case without also knowing the locale of the data.
Since the function does not let you specify a locale, you should only use this function for text written in languages that follow the same orthography as English.
Although it does handle some digraphs, such as the Dutch ij (ij), it does not handle others, such as the Polish dz (ʣ).