Linux – what do I need to do with “man: can’t set the locale; make sure $LC_* and $LANG are correct”

Wikipedia is probably not the best reference for stuff like this. It usually has very simple examples that may not be widely applicable, constructed for understanding concepts more than for practical considerations.

Why not use gnu's own documentation?

http://www.gnu.org/software/gettext/manual/gettext.html#Setting-the-POSIX-Locale

You can set LANGUAGE to "es" (or even "es:fr:en" for a priority list), but LANG would still need to be set to es_MX or something like that. The docs explain it fairly clearly.

A door is a special file in Solaris for bidirectional communication between processes (IPC). Doors provide client processes with a way to call remote code in a server process and fetch the result.

As @HBrujin stated, the Wikipedia article is a good entry point to doors (if that makes sense to you).

As explained by ls man page, doors are identified by the letter D in ls output.