Move declaration of lang_frame_mismatch_warn to header.

Move declaration of lang_frame_mismatch_warn to header.

Also makes it localizable.

gdb/ChangeLog:

2019-10-08  Christian Biesinger  <cbiesinger@google.com>

	* language.c (show_language_command): Pass lang_frame_mismatch_warn
	through _().
	(lang_frame_mismatch_warn): Make const, mark with N_(), and
	move comment...
	* language.h (lang_frame_mismatch_warn): ... here. Also add
	declaration.
	* top.c (lang_frame_mismatch_warn): Remove declaration.
	(check_frame_language_change): Pass lang_frame_mismatch_warn
	through _().