Letter casing functions are available in the core strings
package and always
have Unicode semantics.
lowercase = strings.ToLower(str)
uppercase = strings.ToUpper(str)
titlecase = strings.ToTitle(str) // every character
titlecase = strings.Title(str) // first character of each word
Although there is no case folding function for strings, there is a caseless equality function that uses case folding,
strings.EqualFold(str1, str2)