Compile with explicit language flag when source LANGUAGE property is set

This change was originally made by !4780 (merged), but it was reverted by !5519 (merged) to restore compatibility with pre-3.19 behavior.

Implement the change again, but add policy CMP0119 to make this change while preserving compatibility with existing projects.

Co-Author: @robertmaynard
Fixes: #14516 (closed), #20716 (closed)