Allow setting language of compiler messages (#9486)

* Add a build_language config.yaml option which controls the language
  of compiler messages
* build_language defaults to "C", in which case the compiler messages
  will be in English. This allows Spack log parsing to detect and
  highlight error messages (since the regular expressions to find
  error messages are in English)
* The user can use the default language in their environment by setting
  the build_language config variable to null or ''