For scripting languages, sure. For compiled languages, you can choose whatever version you want, since the end user gets a binary blob instead of source code (and even if they're building from source, their package manager should be able to download the required build dependencies for them).
Where? Java has such short maintainable cycles that Java 7 was dropped before Java 8 became used, and compiled languages (think C, C++, Haskell) always work.
Are there any other languages relevant for non-server usage?
That's nearly never the case, unless you are running uncompiled scripts downloaded from the internet with no version manifest, in which case that's also horrible.
