Off the top of my head, in turkish, `i` doesn't become `I`, it becomes `İ`. And `ı` is the lower case version of `I`

You don't need to decide how to upper or lower case a character to be insensitive to case, though. Treating them all as matching isn't a terrible option.

For example, it depends on the locale if the capitalized form of ß is ß or SS.