(co-creator of GoLang Rob Pike is also co-creator of UTF8 :)
Does storing strings as UTF8 has a practical advantage?
Here is an interesting detailed analysis by a real C# expert:
Of memory and strings | Jon Skeet's coding blog
Compact strings in the CLR · Performance is a Feature!
Go code is UTF-8 encoded – golangspec – Medium
Golang Strings - golangbot.com
Java May Use UTF-8 as its Default Charset - DZone Java