A front end language directory contains the source files of that front end (but not of any runtime libraries, which should be outside the gcc directory). This includes documentation, and possibly some subsidiary programs built alongside the front end. Certain files are special and other parts of the compiler depend on their names:
This file is required in all language subdirectories. See The Front End config-lang.in File, for details of its contents
This file is required in all language subdirectories. See The Front End Make-lang.in File, for details of its contents.
This file registers the set of switches that the front end accepts on the command line, and their --help text. See Option specification files.
This file provides entries for default_compilers in
gcc.cc which override the default of giving an error that a
compiler for that language is not installed.
This file, which need not exist, defines any language-specific tree codes.