Set the language for a single program execution

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.

In this specific case, instead of letting Gnome run dbus-launch to create a random D-Bus address, start dbus-daemon explicitly early in your X session startup and give it a fixed address like unix:path=~/.dbus-$HOSTNAME-$DISPLAY.

Given the information in the bug report, you may even be able to get away with unset DBUS_SESSION_BUS_ADDRESS and let applications find out the bus address from the root window properties.

In the general case, your assessment is correct: all you have is unreliable methods such as ptrace (which may crash the program, or not work due to a security framework such as Apparmor or SELinux) or injecting a shell command (which only works in panes that are currently at a shell prompt). Running a command at each shell prompt (with zsh's preexec or bash's PROMPT_COMMAND) at least doesn't risk breaking stuff.

Another solution would be an LD_PRELOAD library that intercepts getenv calls. This also feels like overkill.

Your best bet is to let the application do the job by creating a level of indirection: arrange for the value of the environment variable to remain valid, and for the application to interpret it in a situation-aware manner. Letting the application look up the D-Bus bus address in the root window properties is an example of this approach.