You shouldn't depend on user environment. This won't work as intended in many cases, e.g. even if user has LANG set, it may not be propagated to su/sudo session; if it is propagated, builds will be inconsistent for example when port is built by different users with different locales. Also, LANG will never be set for packages.

If this is needed for correct built, set it unconditionally (and better set LC_ALL, so even if LANG/LC_* are set, they'll always be overridden). If locale affects something useful, you may add an option instead.

post-extract happens outside SU_CMD even in NO_STAGE times.

Guest illustrations with filenames in SJIS are just fan art not used during the game. English version doesn't have them.

As removing or not installing the files is going to require the same amount of care I won't bother unless there's a better way than:

# -*- encoding: sjis -*-
...
post-extract:
	@${RM} -rf ${WRKSRC}/�Q�X�g��

or

post-extract:
	@${RM} -rf ${WRKSRC}/$$(${ECHO_CMD} ゲスト壁 | \
		${ICONV_CMD} -f UTF-8 -t SJIS-MS)