RFC: Rework Language behaviour

[mkst]

Using this Issue as a bit of a braindump and try to see if we can improve the experience a bit.

1. Some compilers accept an explicit language flag
2. Some compilers will base the behaviour on the file extension (e.g. .c vs .cpp)
3. Some compilers allow language to be overridden (i.e. -x c even if extension is .cpp).
4. Other?

We have a lot of doubled-up compilers, a C++ and C version for example. Can we clean any of this up?