Python <3.3 uses UCS2 or UCS4, depending on the build

Ruby >1.8 lets you choose the encoding

.NET UCS2/UTF-16 (I know the difference, imho if the stdlib has a .size, .length or .count that works on code units instead of code points it's broken... thus I'll mention only UCS2 from now on)

Java UCS2

Clojure UCS2

Scala UCS2

QT UCS2

Haskell String UCS4

Haskell Data.Text UTF-16 (yes, not a naive UCS-2)

Rust UCS4 (last time I checked)

Javascript UCS2

Dart UCS2

PHP Unicode-oblivious

Vala UCS4

Go UTF-8 (but it lets you call len() on strings, and it doesn't return the length of the string, but its size in bytes)

I can't really think of another language that uses UTF-8 internally, are you sure?